Function 模板不起作用，出现错误“没有合适的用户定义的转换”

Question

I am trying to use a container ( std::vector , std::list , std::map , etc...) on a function template but I keep getting an error saying "No suitable user-defined conversion exists"

I tried making a different function template, print_container() with 1 argument, and it works.

#include "stdafx.h"
#include <iostream>
#include <vector>

template<typename T>
using Iterator = typename T::iterator;

template<typename C, typename V>
std::vector<Iterator<C>> find_all(C& container, V value) {
    std::vector<Iterator<C>> res;
    for (auto p = container.begin(); p != container.end(); ++p)
        if ((*p) == value)
            res.push_back(p);
    return res;
}

int main() {
    std::vector<int> vec1 = { 1, 2, 3 };
    std::vector<Iterator<int>> res = find_all(vec1, 1); // does not work
    return 0;
}

find_all() should return a std::vector of iterators with only 1 iterator, the iterator attached to vec1[0] and assign that vector to res .

Answer 1

The problem is in the returned type

std::vector<Iterator<int>> res = find_all(vec1, 1);
//...................^^^ wrong

From that call you obtain a vector of iterators of std::vector<int> , not of int

std::vector<Iterator<std::vector<int>>> res = find_all(vec1, 1);
//...................^^^^^^^^^^^^^^^^  correct    

To avoid this sort of problems, usually you can use auto (starting from C++11)

auto res = find_all(vec1, 1);

Answer 2

The return type is std::vector<Iterator<std::vector<int>>> , not std::vector<Iterator<int>> .

std::vector<Iterator<std::vector<int>>> res = find_all(vec1, 1);

Use of auto is better for cases like this.

auto res = find_all(vec1, 1);

Answer 3

The template argument of the Iterator in this declaration

std::vector<Iterator<int>> res = find_all(v, 1);

Is invalid. The type int has no iterators. See this alias declaration

template<typename T>
using Iterator = typename T::iterator;

You should write either

std::vector<Iterator<std::vector<int>>> res = find_all(v, 1);

or

auto res = find_all(v, 1);