default-arguments

来源： https://stackoverflow.com/questions/35150772/friend-function-template-with-default-template-argument

来源： https://stackoverflow.com/questions/35150772/friend-function-template-with-default-template-argument

问题 Here is a C++ code I wrote to default to user input when arguments are not provided explicitly. I have been learning Python-3.7 for the past week and am trying to achieve a similar functionality. This is the code I tried: def foo(number = int(input())): print(number) foo(2) #defaults to user input but prints the passed parameter and ignores the input foo() #defaults to user input and prints user input This code works, but not quite as intended. You see, when I pass an argument to foo() , it

问题 I wrote the following code (Still wondering about its uses...) to default to user input if the parameter is not passed. #define _CRT_SECURE_NO_WARNINGS #include <iostream> unsigned getInput() { unsigned input; std::cin >> input; return input; } void foo(unsigned number = getInput()) { std::cout << number << "\n"; } int main() { foo(1); //prints 1 foo(); //defaults to user input return 0; } What I wanted to ask was, is there any way we can convert the getInput() function to a lambda? Something

问题 I wrote the following code (Still wondering about its uses...) to default to user input if the parameter is not passed. #define _CRT_SECURE_NO_WARNINGS #include <iostream> unsigned getInput() { unsigned input; std::cin >> input; return input; } void foo(unsigned number = getInput()) { std::cout << number << "\n"; } int main() { foo(1); //prints 1 foo(); //defaults to user input return 0; } What I wanted to ask was, is there any way we can convert the getInput() function to a lambda? Something

问题 Let's say I have this code template<typename T2, typename T = int> struct X { static double f; }; template<typename T> double X<T>::f = 14.0; If I try to compile clang give me the following error nested name specifier 'X::' for declaration does not refer into a class, class template or class template partial specialization and for GCC : error: template definition of non-template 'double X::f' The question is : Why the compiler want us to specialize the struct X like that : template<typename

问题 Let's say I have this code template<typename T2, typename T = int> struct X { static double f; }; template<typename T> double X<T>::f = 14.0; If I try to compile clang give me the following error nested name specifier 'X::' for declaration does not refer into a class, class template or class template partial specialization and for GCC : error: template definition of non-template 'double X::f' The question is : Why the compiler want us to specialize the struct X like that : template<typename

问题 Let's say I have this code template<typename T2, typename T = int> struct X { static double f; }; template<typename T> double X<T>::f = 14.0; If I try to compile clang give me the following error nested name specifier 'X::' for declaration does not refer into a class, class template or class template partial specialization and for GCC : error: template definition of non-template 'double X::f' The question is : Why the compiler want us to specialize the struct X like that : template<typename

问题 I'm using pybind11 to wrap a C++ class method in a conversion lambda "shim" (I must do this because reasons). One of the method's arguments is defaulted in C++. class A { void meow(Eigen::Matrix4f optMat = Eigen::Matrix4f::Identity()); }; In my pybind code I want to preserve this optional parameter: py::class_<A>(m, "A") .def(py::init<>()) .def("meow", [](A& self, Eigen::Matrix4f optMat = Eigen::Matrix4f::Identity()) { return self.meow( optMat ); }); How do I make optMat an optional named

问题 I have this snippet of code which seems to work well: class foo{/* some member variables and functions*/}; void do_somthing(foo x={}){} int main(){ do_somthing(); } I used to use void do_somthing(foo x=foo()){} to default the x argument but I see this way ={} in some book or online example(can not remember). Is it totally ok to use it? Is there any difference between the two methods? 回答1: foo x=foo() is copy initialization, Initializes an object from another object and foo() is value