c++ - Lambda function with no arguments as a default argument for std::function in template -


i'm banging head here, because below code worked hours, until started getting error out of blue.

#include <iostream> #include <string> #include <functional> #include <algorithm>  template<typename ret_t> ret_t get_num(const std::string &prompt = "",             const std::string &error = "",                 const std::function<bool(ret_t)> &condition = [] { return true; })                             //default value of &conditon lambda returns true  {     //....not complete function     ret_t num = 0;     std::cin >> num;      if (condition(num))         return num;      std::cerr << error;     return get_num<ret_t>(prompt, error, condition); }  int main() {     //it works fine if supply third argument     int number = get_num<int>("enter num: ", "bad input! ", [](int num) { return num > 0; });      //this gives me error, while should run      //fine , condition should evaluate true     int number2 = get_num<int>("enter num2: ", "bad input! ");      std::cout << number;      std::cin.ignore();     std::cin.get(); }

everything works fine if specify type of of variable lambda takes (no matter type is):

template<typename ret_t> ret_t get_num(const std::string &prompt = "",             const std::string &error = "",                 const std::function<bool(ret_t)> &condition = [] (ret_t) { return true; })

also, if don't specify default value of &condition lambda, instead if use normal function below, still not work:

bool func() {     return true; }  template<typename ret_t> ret_t get_num(const std::string &prompt = "",             const std::string &error = "",                 const std::function<bool(ret_t)> &condition = func)


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