插入从函数返回的 std::vector [英] Inserting a std::vector as returned from a function

问题描述

我发现自己经常做这样的事情来连接几个从函数(可能是类函数)返回的向量:

I find myself frequently doing something like this to concatenate several vectors that are returned from functions (possibly class functions):

#include <vector>
#include <iostream>
using namespace std;

vector<int> v1;

const vector<int>& F1() {
    cout << "F1 was called" << endl;
    /*Populate v1, which may be an expensive operation*/
    return v1;
}

int main() {
    vector<int> Concat;
    Concat.insert(Concat.end(), F1().begin(), F1().end());
    /*Do something with Concat*/
    return 0;
}

正如我所料，F1() 被调用了两次，如果它是一个昂贵的函数调用，这可能是不可取的.另一种方法是将 F1() 的返回值复制到一个临时向量中，该向量只需要一个函数调用，但会导致复制操作，如果向量很大，这可能是不可取的.我能想到的唯一另一种选择是创建一个指向临时向量的指针，并将 F1() 的返回值分配给它，如下所示:

As I expected, F1() is called twice, which may be undesirable if it is an expensive function call. An alternative is to copy the return value of F1() into a temporary vector which would only require one function call, but would incur a copy operation which might be undesirable if the vector is large. The only other alternative I can think of is to create a pointer to a temporary vector and assign the return value of F1() to it like this:

int main() {
    vector<int> Concat;
    const vector<int>* temp = &F1();
    Concat.insert(Concat.end(), temp->begin(), temp->end());
    /*Do something with Concat*/
    return 0;
}

这真的是最好的解决方案吗?使用临时变量似乎很麻烦，尤其是当我需要连接多个向量时.我也觉得应该有一种方法可以使用引用而不是指针来做到这一点.有什么建议吗?

Is this really the best solution? The use of a temporary variable seems cumbersome, especially if I need to concatenate several vectors. I also feel like there should be a way to do this using references instead of pointers. Any suggestions?

推荐答案

最好的解决方案不是一开始就直接使用vector而是OutputIterators和>std::back_inserter.

The best solution is not to use vector directly in the first place but OutputIterators and std::back_inserter.

template <typename OutputIterator>
OutputIterator F1( OutputIterator out )
{
    cout << "F1 was called" << endl;
    /* Insert stuff via *out++ = ...; */
    *out++ = 7;
    return out;
}

int main()
{
    std::vector<int> Concat;
    // perhaps reserve some moderate amount of storage to avoid reallocation

    F1( std::back_inserter(Concat) );
    F1( std::back_inserter(Concat) );
}

演示.这样可以实现最大的效率和灵活性.

Demo. This way maximum efficiency and flexibility are achieved.

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