每个 C++开发人员都应该使用的10个 C++ 11特性
目录
- auto
- nullptr
- Range-Based for Loops
- Override and Final
- Strongly-typed Enums
- Smart Pointers
- Lambdas
- Non-Member begin() and end()
- static_assert and Type Traits
- Move Semantics
auto
Before C++11, the auto keyword was used for storage duration specification. In the new standard, its purpose was changed towards type inference. auto is now a sort of placeholder for a type, telling the compiler it has to deduce the actual type of a variable that is being declared from its initializer. It can be used when declaring variables in different scopes such as namespaces, blocks or initialization statement of for loops.
auto i = 42; // i is an int
auto l = 42LL; // l is an long long
auto p = new foo(); // p is a foo*
Using auto usually means less code (unless your type is int which is one letter shorter). Think of iterators in STL that you always had to write while iterating over containers. It makes obsolete creating typedefs just for the sake of simplicity.
std::map<std::string, std::vector<int>> map;
for(auto it = begin(map); it != end(map); ++it)
{
}
You should note that auto cannot be used as the return type of a function. However, you can use auto in place of the return type of function, but in this case the function must have a trailing return type. In this case, auto does not tell the compiler it has to infer the type, it only instructs it to look for the return type at the end of the function. In the example below, the return type of function compose is the return type of operator+ that sums values of types T1 and T2.
template <typename T1, typename T2>
auto compose(T1 t1, T2 t2) -> decltype(t1 + t2)
{
return t1+t2;
}
auto v = compose(2, 3.14); // v's type is double
nullptr
Zero used to be the value of null pointers, and that has drawbacks due to the implicit conversion to integral types. The keyword nullptr denotes a value of type std::nullptr_t that represents the null pointer literal. Implicit conversions exists from nullptr to null pointer value of any pointer type and any pointer-to-member types, but also to bool (as false). But no implicit conversion to integral types exist.
void foo(int* p) {}
void bar(std::shared_ptr<int> p) {}
int* p1 = NULL;
int* p2 = nullptr;
if(p1 == p2)
{
}
foo(nullptr);
bar(nullptr);
bool f = nullptr;
int i = nullptr; // error: A native nullptr can only be converted to bool or,
// using reinterpret_cast, to an integral type
For backward compatibility, 0 is still a valid null pointer value.
基于范围的循环
C++11 augmented the for statement to support the “foreach” paradigm of iterating over collections. In the new form, it is possible to iterate over C-like arrays, initializer lists and anything for which the non-member begin() and end() functions are overloaded.
This for each for is useful when you just want to get and do something with the elements of a collection/array and don’t care about indexes, iterators or number of elements.
std::map<std::string, std::vector<int>> map;
std::vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
map["one"] = v;
for(const auto& kvp : map)
{
std::cout << kvp.first << std::endl;
for(auto v : kvp.second)
{
std::cout << v << std::endl;
}
}
int arr[] = {1,2,3,4,5};
for(int& e : arr)
{
e = e*e;
}
Override 和 Final
I always found the virtual methods badly designed in C++ because there wasn’t (and still isn’t) a mandatory mechanism to mark virtual methods as overridden in derived classes. The virtual keyword is optional and that makes reading code a bit harder, because you may have to look through the top of the hierarchy to check if the method is virtual. I have always used, and encouraged people to use the virtual keyword on derived classes also, to make the code easier to read. However, there are subtle errors that can still arise. Take for instance, the following example:
class B
{
public:
virtual void f(short) {std::cout << "B::f" << std::endl;}
};
class D : public B
{
public:
virtual void f(int) {std::cout << "D::f" << std::endl;}
};
D::f is supposed to override B::f. However, the signatures differ, one takes a short, one takes an int, therefore B::f is just another method with the same name (and overload) and not an override. You may call f() through a pointer to B and expect to print D::f, but it’s printing B::f.
Here is another subtle error: the parameters are the same, but the method in the base class is marked const, while the method in the derived is not.
class B
{
public:
virtual void f(int) const {std::cout << "B::f " << std::endl;}
};
class D : public B
{
public:
virtual void f(int) {std::cout << "D::f" << std::endl;}
};
Again, these two are overloads and not overrides, so if you call f() through a pointer to B, it will print B::f and not D::f.
Fortunately, there is now a way to describe your intentions. Two new special identifiers (not keywords) have been added: override, to indicate that a method is supposed to be an override of a virtual method in a base class, and final, to indicate that a derived class shall not override a virtual method. The first example would become:
class B
{
public:
virtual void f(short) {std::cout << "B::f" << std::endl;}
};
class D : public B
{
public:
virtual void f(int) override {std::cout << "D::f" << std::endl;}
};
This now triggers a compiler error (the same error you’d get for the second example too, if using the override specifier):
'D::f' : method with override specifier 'override' did not override any base class methods
On the other hand, if you intend to make a method impossible to override any more (down the hierarchy), mark it as final. That can be in the base class, or any derived class. If it’s in a derived classe, you can use both the override and final specifiers.
class B
{
public:
virtual void f(int) {std::cout << "B::f" << std::endl;}
};
class D : public B
{
public:
virtual void f(int) override final {std::cout << "D::f" << std::endl;}
};
class F : public D
{
public:
virtual void f(int) override {std::cout << "F::f" << std::endl;}
};
function declared as 'final' cannot be overridden by 'F::f'
强类型枚举
“Traditional” enums in C++ have some drawbacks: they export their enumerators in the surrounding scope (which can lead to name collisions, if two different enums in the same have scope define enumerators with the same name), they are implicitly converted to integral types and cannot have a user-specified underlying type.
These issues have been fixed in C++ 11 with the introduction of a new category of enums, called strongly-typed enums. They are specified with the enum class keywords. They no longer export their enumerators in the surrounding scope, are no longer implicitly converted to integral types and can have a user-specified underlying type (a feature also added for traditional enums).
enum class Options {None, One, All};
Options o = Options::All;
智能指针
There have been tons of articles written on this subject, therefore I just want to mention the smart pointers with reference counting and auto releasing of owned memory that are available:
- unique_ptr: should be used when ownership of a memory resource does not have to be shared (it doesn’t have a copy constructor), but it can be transferred to another
unique_ptr(move constructor exists). - shared_ptr: should be used when ownership of a memory resource should be shared (hence the name).
- weak_ptr: holds a reference to an object managed by a
shared_ptr, but does not contribute to the reference count; it is used to break dependency cycles (think of a tree where the parent holds an owning reference (shared_ptr) to its children, but the children also must hold a reference to the parent; if this second reference was also an owning one, a cycle would be created and no object would ever be released).
On the other hand, the auto_ptr is obsolete and should no longer be used.
When you should use unique_ptr and when you should use shared_ptr depends on the ownership requirements and I recommend reading this discussion.
The first example below shows unique_ptr. If you want to transfer ownership of an object to another unique_ptr, use std::move (I’ll discuss this function in the last paragraph). After the ownership transfer, the smart pointer that ceded the ownership becomes null and get() returns nullptr.
void foo(int* p)
{
std::cout << *p << std::endl;
}
std::unique_ptr<int> p1(new int(42));
std::unique_ptr<int> p2 = std::move(p1); // transfer ownership
if(p1)
foo(p1.get());
(*p2)++;
if(p2)
foo(p2.get());
The second example shows shared_ptr. Usage is similar, though the semantics are different since ownership is shared.
void foo(int* p)
{
}
void bar(std::shared_ptr<int> p)
{
++(*p);
}
std::shared_ptr<int> p1(new int(42));
std::shared_ptr<int> p2 = p1;
bar(p1);
foo(p2.get());
The first declaration is equivalent to this one.
auto p3 = std::make_shared<int>(42);
make_shared is a non-member function and has the advantage of allocating memory for the shared object and the smart pointer with a single allocation, as opposed to the explicit construction of a shared_ptr via the contructor, that requires at least two allocations. In addition to possible overhead, there can be situations where memory leaks can occur because of that. In the next example, memory leaks could occur if seed() throws an error.
void foo(std::shared_ptr<int> p, int init)
{
*p = init;
}
foo(std::shared_ptr<int>(new int(42)), seed());
No such problem exists if using make_shared. The third sample shows usage of weak_ptr. Notice that you must always get a shared_ptr to the referred object by calling lock(), in order to access the object.
auto p = std::make_shared<int>(42);
std::weak_ptr<int> wp = p;
{
auto sp = wp.lock();
std::cout << *sp << std::endl;
}
p.reset();
if(wp.expired())
std::cout << "expired" << std::endl;
If you try to lock on an expired weak_ptr (the object is weakly reference has been released), you get an empty shared_ptr.
匿名函数
Anonymous functions, called lambda, have been added to C++ and quickly rose to prominence. It is a powerful feature borrowed from functional programming, that in turn enabled other features or powered libraries. You can use lambdas wherever a function object or a functor or a std::function is expected. You can read about the syntax here.
std::vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
std::for_each(std::begin(v), std::end(v), [](int n) {std::cout << n << std::endl;});
auto is_odd = [](int n) {return n%2==1;};
auto pos = std::find_if(std::begin(v), std::end(v), is_odd);
if(pos != std::end(v))
std::cout << *pos << std::endl;
A bit trickier are recursive lambdas. Imagine a lambda that represents a Fibonacci function. If you attempt to write it using auto, you get a compilation error:
auto fib = [&fib](int n) {return n < 2 ? 1 : fib(n-1) + fib(n-2);};
error C3533: 'auto &': a parameter cannot have a type that contains 'auto'
error C3531: 'fib': a symbol whose type contains 'auto' must have an initializer
error C3536: 'fib': cannot be used before it is initialized
error C2064: term does not evaluate to a function taking 1 arguments
The problem is auto means the type of the object is inferred from its initializer, yet the initializer contains a reference to it, therefore needs to know its type. This is a cyclic problem. The key is to break this dependency cycle and explicitly specify the function’s type using std::function.
std::function<int(int)> lfib = [&lfib](int n) {return n < 2 ? 1 : lfib(n-1) + lfib(n-2);};
非成员 begin()和 end()
You probably noticed that I have used in the samples above non-member begin() and end() functions. These are a new addition to the standard library, promoting uniformity, consistency and enabling more generic programming. They work with all STL containers, but more than that, they are overloadable, so they can be extended to work with any type. Overloads for C-like arrays are also provided.
Let’s take, for instance, the previous example where I was printing a vector and then looking for its first odd element. If the std::vector was instead a C-like array, the code might have looked like this:
int arr[] = {1,2,3};
std::for_each(&arr[0], &arr[0]+sizeof(arr)/sizeof(arr[0]), [](int n)
{std::cout << n << std::endl;});
auto is_odd = [](int n) {return n%2==1;};
auto begin = &arr[0];
auto end = &arr[0]+sizeof(arr)/sizeof(arr[0]);
auto pos = std::find_if(begin, end, is_odd);
if(pos != end)
std::cout << *pos << std::endl;
With non-member begin() and end(), it could be put as this:
int arr[] = {1,2,3};
std::for_each(std::begin(arr), std::end(arr), [](int n) {std::cout << n << std::endl;});
auto is_odd = [](int n) {return n%2==1;};
auto pos = std::find_if(std::begin(arr), std::end(arr), is_odd);
if(pos != std::end(arr))
std::cout << *pos << std::endl;
This is basically identical code to the std::vector version. That means we can write a single generic method for all types supported by begin() and end().
template <typename Iterator>
void bar(Iterator begin, Iterator end)
{
std::for_each(begin, end, [](int n) {std::cout << n << std::endl;});
auto is_odd = [](int n) {return n%2==1;};
auto pos = std::find_if(begin, end, is_odd);
if(pos != end)
std::cout << *pos << std::endl;
}
template <typename C>
void foo(C c)
{
bar(std::begin(c), std::end(c));
}
template <typename T, size_t N>
void foo(T(&arr)[N])
{
bar(std::begin(arr), std::end(arr));
}
int arr[] = {1,2,3};
foo(arr);
std::vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
foo(v);
static_assert 和 Type Traits
static_assert performs an assertion check at compile-time. If the assertion is true, nothing happens. If the assertion is false, the compiler displays the specified error message.
template <typename T, size_t Size>
class Vector
{
static_assert(Size < 3, "Size is too small");
T _points[Size];
};
int main()
{
Vector<int, 16> a1;
Vector<double, 2> a2;
return 0;
}
error C2338: Size is too small
see reference to class template instantiation 'Vector<T,Size>' being compiled
with
[
T=double,
Size=2
]
static_assert becomes more useful when used together with type traits. These are a series of classes that provide information about types at compile time. They are available in the header. There are several categories of classes in this header: helper classes, for creating compile-time constants, type traits classes, to get type information at compile time, and type transformation classes, for getting new types by applying transformation on existing types.
In the following example, function add is supposed to work only with integral types.
template <typename T1, typename T2>
auto add(T1 t1, T2 t2) -> decltype(t1 + t2)
{
return t1 + t2;
}
However, there are no compiler errors if one writes:
std::cout << add(1, 3.14) << std::endl;
std::cout << add("one", 2) << std::endl;
The program actually prints 4.14 and “e”. But if we add some compile-time asserts, both these lines would generate compiler errors.
template <typename T1, typename T2>
auto add(T1 t1, T2 t2) -> decltype(t1 + t2)
{
static_assert(std::is_integral<T1>::value, "Type T1 must be integral");
static_assert(std::is_integral<T2>::value, "Type T2 must be integral");
return t1 + t2;
}
error C2338: Type T2 must be integral
see reference to function template instantiation 'T2 add<int,double>(T1,T2)' being compiled
with
[
T2=double,
T1=int
]
error C2338: Type T1 must be integral
see reference to function template instantiation 'T1 add<const char*,int>(T1,T2)' being compiled
with
[
T1=const char *,
T2=int
]
转移语义
This is yet another important and well covered topic from C++11, that one could write a series of articles, not just a paragraph. Therefore, I will not get into too many details, but encourage you to find additional readings, if you’re not already familiar with the topic.
C++11 has introduced the concept of rvalue references (specified with &&) to differentiate a reference to an lvalue or an rvalue. An lvalue is an object that has a name, while an rvalue is an object that does not have a name (a temporary object). The move semantics allow modifying rvalues (previously considered immutable and indistinguishable from const T& types).
A C++ class/struct used to have some implicit member functions: default constructor (only if another constructor is not explicitly defined) and copy constructor, a destructor and a copy assignment operator. The copy constructor and the copy assignment operator perform a bit-wise (or shallow) copy, i.e., copying the variables bitwise. That means if you have a class that contains pointers to some objects, they just copy the value of the pointers and not the objects they point to. This might be OK in some cases, but for many cases, you actually want a deep-copy, meaning that you want to copy the objects pointers refer to, and not the values of the pointers. In this case, you have to explicitly write copy constructor and copy assignment operator to perform a deep-copy.
What if the object you initialize or copy from is an rvalue (a temporary). You still have to copy its value, but soon after the rvalue goes away. That means an overhead of operations, including allocations and memory copying that after all, should not be necessary.
Enter the move constructor and move assignment operator. These two special functions take a T&& argument, which is an rvalue. Knowing that fact, they can modify the object, such as “stealing” the objects their pointers refer to. For instance, a container implementation (such as a vector or a queue) may have a pointer to an array of elements. When an object is instantiating from a temporary, instead of allocating another array, copying the values from the temporary, and then deleting the memory from the temporary when that is destroyed, we just copy the value of the pointer that refers to the allocated array, thus saving an allocation, copying a sequence of elements, and a later de-allocation.
The following example shows a dummy buffer implementation. The buffer is identified by a name (just for the sake of showing a point revealed below), has a pointer (wrapper in an std::unique_ptr) to an array of elements of type T and variable that tells the size of the array.
template <typename T>
class Buffer
{
std::string _name;
size_t _size;
std::unique_ptr<T[]> _buffer;
public:
// default constructor
Buffer():
_size(16),
_buffer(new T[16])
{}
// constructor
Buffer(const std::string& name, size_t size):
_name(name),
_size(size),
_buffer(new T[size])
{}
// copy constructor
Buffer(const Buffer& copy):
_name(copy._name),
_size(copy._size),
_buffer(new T[copy._size])
{
T* source = copy._buffer.get();
T* dest = _buffer.get();
std::copy(source, source + copy._size, dest);
}
// copy assignment operator
Buffer& operator=(const Buffer& copy)
{
if(this != ©)
{
_name = copy._name;
if(_size != copy._size)
{
_buffer = nullptr;
_size = copy._size;
_buffer = _size > 0 > new T[_size] : nullptr;
}
T* source = copy._buffer.get();
T* dest = _buffer.get();
std::copy(source, source + copy._size, dest);
}
return *this;
}
// move constructor
Buffer(Buffer&& temp):
_name(std::move(temp._name)),
_size(temp._size),
_buffer(std::move(temp._buffer))
{
temp._buffer = nullptr;
temp._size = 0;
}
// move assignment operator
Buffer& operator=(Buffer&& temp)
{
assert(this != &temp); // assert if this is not a temporary
_buffer = nullptr;
_size = temp._size;
_buffer = std::move(temp._buffer);
_name = std::move(temp._name);
temp._buffer = nullptr;
temp._size = 0;
return *this;
}
};
template <typename T>
Buffer<T> getBuffer(const std::string& name)
{
Buffer<T> b(name, 128);
return b;
}
int main()
{
Buffer<int> b1;
Buffer<int> b2("buf2", 64);
Buffer<int> b3 = b2;
Buffer<int> b4 = getBuffer<int>("buf4");
b1 = getBuffer<int>("buf5");
return 0;
}
The default copy constructor and copy assignment operator should look familiar. What’s new to C++11 is the move constructor and move assignment operator, implemented in the spirit of the aforementioned move semantics. If you run this code, you’ll see that when b4 is constructed, the move constructor is called. Also, when b1 is assigned a value, the move assignment operator is called. The reason is the value returned by getBuffer() is a temporary, i.e., an rvalue.
You probably noticed the use of std::move in the move constructor, when initializing the name variable and the pointer to the buffer. The name is actually a string, and std::string also implements move semantics. Same for the std::unique_ptr. However, if we just said _name(temp._name) the copy constructor would have been called. For _buffer, that would not have been even possible because std::unique_ptr does not have a copy constructor. But why wasn’t the move constructor for std::string called in this case? Because even if the object the move constructor for Buffer is called with is an rvalue, inside the constructor, it is actually an lvalue. Why? Because it has a name, “temp” and a named object is an lvalue. To make it again an rvalue (and be able to invoke the appropriate move constructor), one must use std::move. This function just turns an lvalue reference into an rvalue reference.
UPDATE: Though the purpose of this example was to show how move constructor and move assignment operator should be implemented, the exact details of an implementation may vary. An alternative implementation was provided by Member 7805758 in the comments. To be easier to see it, I will show it here:
template <typename T>
class Buffer
{
std::string _name;
size_t _size;
std::unique_ptr<T[]> _buffer;
public:
// constructor
Buffer(const std::string& name = "", size_t size = 16):
_name(name),
_size(size),
_buffer(size? new T[size] : nullptr)
{}
// copy constructor
Buffer(const Buffer& copy):
_name(copy._name),
_size(copy._size),
_buffer(copy._size? new T[copy._size] : nullptr)
{
T* source = copy._buffer.get();
T* dest = _buffer.get();
std::copy(source, source + copy._size, dest);
}
// copy assignment operator
Buffer& operator=(Buffer copy)
{
swap(*this, copy);
return *this;
}
// move constructor
Buffer(Buffer&& temp):Buffer()
{
swap(*this, temp);
}
friend void swap(Buffer& first, Buffer& second) noexcept
{
using std::swap;
swap(first._name , second._name);
swap(first._size , second._size);
swap(first._buffer, second._buffer);
}
};
总结
There are many more things to say about C++11; this was just one of many possible beginnings. This article presented a series of core language and standard library features that every C++ developer should use. However, I recommend you additional readings, at least for some of these features.