为什么std :: transform不将std :: string向量转换为unsigned int向量？

Question

As described in the title, I am trying to convert a vector of std::string to unsigned int . But I am getting a segmentation fault. Here's my code:

#include<iostream>
#include<string>
#include<vector>
#include<sstream>

int main() {
    unsigned int N = 3;
    std::string array_string = "2 5 8";
    std::vector<unsigned int> A;

    std::istringstream array_stream(array_string);

    std::vector<std::string> array {
        std::istream_iterator<std::string>{array_stream},
        std::istream_iterator<std::string>{}
    };
    A.clear(); A.reserve(N);
    std::transform(array.begin(), array.end(), A.begin(), [] (const std::string& str) {
        return std::stoi(str);
    });
    for(std::vector<unsigned int>::iterator it = A.begin(); it != A.end(); it++) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;
    std::cout << A.size() << std::endl;
    return 0;
}

OTOH, changing A.begin() to std::back_inserter(A) in the call to std::transform works. Is this because A.begin() fails when A is empty?

Answer 1

Is this because A.begin() fails when A is empty?

Yes, that's why. Your call to reserve doesn't change the fact that A is empty, it is still empty afterwards. Calling reserve only changes the capacity , ie subsequent insertions will not allocate any memory.

If you want to use A.begin() , you have to call resize , as that will actually change the size of A :

A.resize(N); //Resize array
std::transform(array.begin(), array.end(), A.begin(), [] (const std::string& str) {
    return std::stoi(str);
});