C++ 向量，返回与参数 [英] C++ vector, return vs. parameter

问题描述

可能的重复:
如何返回对象"在 C++ 中

我想知道以下三种方法是否有区别:

I am wondering if there is a difference between the three following approaches:

void FillVector_1(vector<int>& v) {
    v.push_back(1); // lots of push_backs!
}

vector<int> FillVector_2() {
    vector<int> v;
    v.push_back(1); // lots of push_backs!
    return v;
}

vector<int> FillVector_3() {
    int tab[SZ] = { 1, 2, 3, /*...*/ };
    return vector<int>(tab, tab + SZ);
}

推荐答案

最大的区别是第一种方式是附加到现有内容，而另外两种方式填充一个空向量.:)

The biggest difference is that the first way appends to existing contents, whereas the other two fill an empty vector. :)

我觉得你要找的关键词是返回值优化，应该比较常见(使用 G++ 你必须专门关闭它以防止它被应用).也就是说，如果用法是这样的:

I think the keyword you are looking for is return value optimization, which should be rather common (with G++ you'll have to turn it off specifically to prevent it from being applied). That is, if the usage is like:

vector<int> vec = fill_vector();

那么可能很容易没有副本(而且该功能更易于使用).

then there might quite easily be no copies made (and the function is just easier to use).

如果您正在使用现有向量

If you are working with an existing vector

vector<int> vec;
while (something)
{
    vec = fill_vector();
    //do things
}

然后使用 out 参数将避免在循环中创建向量和复制数据.

then using an out parameter would avoid creation of vectors in a loop and copying data around.

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