c++ - What does A a() mean? -

this question has answer here:

• most vexing parse 1 answer

consider code:

#include<iostream> using namespace std;  class {     public:     a():age(12){}     int age; };  int main() {     a();     cout << a.age << endl;     return 0; } 

when compile using g++, error:

you can not see member age, because not class a()

can explain me? a a()?

this line

a a(); 

declares function named a, returning a no arguments. (see most vexing parse).

what want is

a = a(); // value-initialization a{}; // same valid in c++11 (and not supported msvs) 

or

a a; // default initialization 

c++11, §8.5/10

note: since () not permitted syntax initializer,

x a();

is not declaration of value-initialized object of class x, declaration of function taking no argument , returning x.

for class, value-initialization == default-initialization (at least outcome). see answer here: c++: initialization of int variables implicit constructor infos on value- vs. default-initialization pod or built-in types.