为什么这个程序有这种行为（push_back）？

Question

The code:

// test2.cpp

#include <vector>
#include <iostream>

struct test_class
{
    test_class() = default;

    test_class(const test_class& t)
    {
        std::cout << "Copied" << std::endl;
    }
};

int main()
{
    test_class a;
    std::vector<test_class> v;

    for (int i = 0; i < 5; ++i) {
        v.push_back(a);
        std::cout << std::endl;
    }
}

The behaviour:

$ g++ --version | grep g++
g++ (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2
$ g++ -std=c++11 test2.cpp
$ ./a.out
Copied

Copied
Copied

Copied
Copied
Copied

Copied

Copied
Copied
Copied
Copied
Copied

Each push_back performs an "undefined" number of copies (where only one copy must be performed).

What is going on here?

Answer 1

The vector allocates continuous memory just like an array. If there is no more space at the end of the memory it has to reallocate the whole vector. After this it will copy the elements from the old place to the new and delete the old.

You can initialize it to be able to hold at least 5 elements, so there wil be no memory allocation and copy in your example:

std::vector<test_class> v(5);

Answer 2

push_back可能导致vector增长超出其分配的存储空间，从而导致重新分配，从而导致内容被复制。