向量重新分配使用副本而不是移动构造函数 [英] Vector reallocation uses copy instead of move constructor
问题描述
您好,我创建了一个带有noexcept的类Foo,使用gcc 4.7移动构造函数,并将矢量保留大小设置为2,以便在添加第三项时它将不得不重新分配大小。看起来这样做是在调用复制构造函数而不是移动构造函数。我在这里错过了什么?
#include< vector>
#include< iostream>
class Foo
{
public:
Foo(int x):data_(x)
{
std :: cout< < 构造<<的std :: ENDL;
}
〜Foo()
{
std :: cout<< 破坏<<的std :: ENDL;
}
Foo& operator =(const Foo&)= default;
Foo& operator =(Foo&& amp;
Foo(Foo& other)noexcept:data_(std :: move(other.data_))
{
std :: cout<< 移动构建<<的std :: ENDL;
}
Foo(const Foo& other)noexcept:data_(other.data_)
{
std :: cout<< 复制构建<<的std :: ENDL;
}
private:
int data_;
};
int main(int argc,char * argv [])
{
std :: vector< Foo> v;
v.reserve(2);
v.emplace_back(1);
std :: cout<< 添加了1<<的std :: ENDL;
v.emplace_back(2);
std :: cout<< 添加2<<的std :: ENDL;
v.emplace_back(3);
std :: cout<< 添加3<<的std :: ENDL;
std :: cout<< v size:<< v.size()<<的std :: ENDL;
}
输出:
<$
构建
增加1
构建
增加2
构建
复制构建
复制构造
破坏
破坏
加入3
v size:3
破坏
破坏
破坏
经过GCC 4.7和4.8的修改之后,它似乎确实是4.7中的一个bug,仅当类的析构函数 标记为
noexcept
时才会出现: <$ $ {
Foo(){}
〜Foo()noexcept {}
Foo(Foo&& amp;)noexcept {std :: cout << 移动构造函数<<的std :: ENDL; }
Foo(const Foo&)noexcept {std :: cout<< 复制构造函数<<的std :: ENDL; }
};
int main(){
std :: vector< Foo> v;
v.reserve(2);
v.emplace_back();
v.emplace_back();
v.emplace_back();
$ / code>
GCC 4.7显示:
移动构造函数
移动构造函数
如果我们从析构函数中删除 noexcept
:
struct Foo {
)Foo(){}
〜Foo(){}
Foo(Foo&&)& 移动构造函数<<的std :: ENDL; }
Foo(const Foo&)noexcept {std :: cout<< 复制构造函数<<的std :: ENDL; }
};
GCC 4.7显示:
复制构造函数
复制构造函数
GCC 4.8使用移动构造函数在这两种情况下。
Hi I created a class Foo with a noexcept move constructor using gcc 4.7 and set the vector reserve size to 2 so that it would have to reallocate the size when adding the 3rd item. It seems it is calling the copy constructor instead of the move constructor when doing this. Am I missing something here?
#include <vector>
#include <iostream>
class Foo
{
public:
Foo(int x) : data_(x)
{
std::cout << " constructing " << std::endl;
}
~Foo()
{
std::cout << " destructing " << std::endl;
}
Foo& operator=(const Foo&) = default;
Foo& operator=(Foo&&) = default;
Foo(Foo&& other) noexcept : data_(std::move(other.data_))
{
std::cout << " Move constructing " << std::endl;
}
Foo(const Foo& other) noexcept : data_(other.data_)
{
std::cout << " Copy constructing " << std::endl;
}
private:
int data_;
};
int main ( int argc, char *argv[])
{
std::vector<Foo> v;
v.reserve(2);
v.emplace_back(1);
std::cout << "Added 1" << std::endl;
v.emplace_back(2);
std::cout << "Added 2" << std::endl;
v.emplace_back(3);
std::cout << "Added 3" << std::endl;
std::cout << "v size: " << v.size() << std::endl;
}
output:
constructing
Added 1
constructing
Added 2
constructing
Copy constructing
Copy constructing
destructing
destructing
Added 3
v size: 3
destructing
destructing
destructing
After tinkering with it a bit with both GCC 4.7 and 4.8, it seems that it is indeed a bug in 4.7, which only appears when the class' destructor is not marked noexcept
:
struct Foo {
Foo() {}
~Foo() noexcept {}
Foo(Foo&&) noexcept { std::cout << "move constructor" << std::endl; }
Foo(const Foo&) noexcept { std::cout << "copy constructor" << std::endl; }
};
int main() {
std::vector<Foo> v;
v.reserve(2);
v.emplace_back();
v.emplace_back();
v.emplace_back();
}
GCC 4.7 displays:
move constructor
move constructor
If we remove noexcept
from the destructor:
struct Foo {
Foo() {}
~Foo() {}
Foo(Foo&&) noexcept { std::cout << "move constructor" << std::endl; }
Foo(const Foo&) noexcept { std::cout << "copy constructor" << std::endl; }
};
GCC 4.7 displays:
copy constructor
copy constructor
GCC 4.8 uses the move constructor in both cases.
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