C++: bizarre occurrence of "Request for member X of Y which is of non-class type Z"
c++, most-vexing-parse
Solution
It's a case of most vexing parse where the compiler interprets `A<B> a2(B(v))` as the declaration of a function. Such that:
`A<B>` is the return type `a2` is the function name `B` is the type of the argument `v` is the argument name
So, when you are doing
std::cout << a1.y.u << " " << a2.y.u << std::endl;
The compiler does not think of `a2.y.u` as a class, that's why you are getting the `non-class type` error.
Also, since double parenthesis aren't allowed in a function declaration, the version `A<B> a2((B(v)));` works because the compiler doesn't interprets it as a function declaration anymore, but as a variable declaration.
Problem
The following program, compiled with g++ 4.6, yields the error ``` request for member ‘y’ in ‘a2’, which is of non-class type ‘A<B>(B)’ ``` at its last line: ``` #include <iostream> template <class T> class A { public: T y; A(T x):y(x){} }; class B { public: int u; B(int v):u(v){} }; int main() { int v = 10; B b1(v); //works A<B> a1(b1); //does not work (the error is when a2 is used) A<B> a2(B(v)); //works //A<B> a2((B(v))); std::cout << a1.y.u << " " << a2.y.u << std::endl; } ``` As can be seen from the working variant included in the code, adding parentheses around the arguments of the constructor of A solves the problem. I have seen some related errors caused by a the interpretation of a constructor invocation as a function declaration, like when creating an object with no argument to its constructor, but with braces: ``` myclass myobj(); ``` but it seems to me that ``` A<B> a2(B(v)); ``` cannot be interpreted as a function declaration. Someone can explain to me what is happening?