全局与局部函数对象

Question

I have a function that returns another function, which captures the parameter of the wrapper function. the local version works, but the global version doesn't, and I don't understand why:

#include <iostream>
#include <sstream>
#include <iomanip>


const auto& parseDateTimeWithFormat = [](const std::string& formatStr) {
    return [&formatStr](const std::string& dateTimeStr) {
        std::cout << formatStr << std::endl;
        tm t = {};
        std::istringstream ss(dateTimeStr);
        ss >> std::get_time(&t, formatStr.c_str());
        const int timestamp = (int)mktime(&t);
        return timestamp;
    };
};


const auto& parseDateTime1 = parseDateTimeWithFormat("%Y-%m-%dT%H:%M:%SZ");


int main(int argc, const char* argv[]) {
    int ts1 = parseDateTime1("2018-10-08T10:09:08Z");
    std::cout << ts1 << std::endl;

    const auto& parseDateTime2 = parseDateTimeWithFormat("%Y-%m-%dT%H:%M:%SZ");
    int ts2 = parseDateTime2("2018-10-08T10:09:08Z");
    std::cout << ts2 << std::endl;

    return 0;
}

output:

(empty string)
-1
%Y-%m-%dT%H:%M:%SZ
1538989748

also, when capturing formatStr by value instead of by reference, the global version works, too.

Answer 1

Your local version might "work" but it also suffers from the same thing as the global version, which is undefined behavior. In all cases when you call parseDateTimeWithFormat you give a string literal. Since that isn't a std::string a temporary one is created for you. That string is what you capture in

return [&formatStr](const std::string& dateTimeStr) {
    std::cout << formatStr << std::endl;
    tm t = {};
    std::istringstream ss(dateTimeStr);
    ss >> std::get_time(&t, formatStr.c_str());
    const int timestamp = (int)mktime(&t);
    return timestamp;
};

and return out of the lambda. Unfornetly, as soon as the expresion ends where you called parseDateTimeWithFormat that temporary is destroyed and you are left with a dangling reference to formatStr . The fix, like you found, is to capture by value so the lambda has it's own copy and you don't try to refer to something that doesn't exist anymore.