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std::string api_key_ = "123456789-5c51509f-8c5c5dc2-b6557";
std::pair<std::string_view, std::string_view> yy = std::make_pair("AccessKeyId", api_key_);
std::cout << "yy.second:" << yy.second << std::endl;
compile with c++17 this will output:
yy.second:9-5c51509f-8c5c5dc2-b6557
while this is good without deduction
std::string api_key_ = "123456789-5c51509f-8c5c5dc2-b6557";
std::string_view sv = api_key_;
std::cout << "sv:" << sv << std::endl;
std::pair<std::string_view, std::string_view> xx = std::make_pair<std::string_view, std::string_view>("AccessKeyId", api_key_);
std::cout << "xx.second:" << xx.second << std::endl;
output:
sv:123456789-5c51509f-8c5c5dc2-b6557
xx.second:123456789-5c51509f-8c5c5dc2-b6557
Can anybody explain? Thanks.
the complete code is as below:
#include <unordered_map>
#include <iostream>
#include <variant>
#include <deque>
#include <vector>
int main() {
    std::string api_key_ = "123456789-5c51509f-8c5c5dc2-b6557";
    std::string_view sv = api_key_;
    std::cout << "sv:" << sv << std::endl;
    std::pair<std::string_view, std::string_view> xx = std::make_pair<std::string_view, std::string_view>("AccessKeyId", api_key_);
    std::cout << "xx.second:" << xx.second << std::endl;
    std::pair<std::string_view, std::string_view> yy = std::make_pair("AccessKeyId", api_key_);
    std::cout << "yy.second:" << yy.second << std::endl;
compile and output:
$ g++ ./try2.cpp -std=c++17
$./a.out
sv:123456789-5c51509f-8c5c5dc2-b6557
xx.second:123456789-5c51509f-8c5c5dc2-b6557
yy.second:9-5c51509f-8c5c5dc2-b6557
std::make_pair("AccessKeyId", api_key_); returns a temproray std::pair whose 2nd element is an std::string. Then yy is initialized from the temporary and its 2nd element std::string_view is constructed from the std::string of the temporary std::pair (but not api_key_). After the full expression the temporary std::pair gets destroyed, left yy's 2nd element holding a dangling pointer.
std::make_pair<std::string_view, std::string_view>("AccessKeyId", api_key_) returns a temporary std::pair too, but its 2nd element is an std::string_view constructed from api_key_. Then xx's 2nd element is initialized from the std::string_view of the temporary std::pair and referring to api_key_ too, that's why it's valid.
        
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