Why does the compiler fail to compile when invoking a template member function within a template function?

Template Member Function Invocation within Template Functions

The code snippet provided demonstrates an error encountered when invoking a template member function from within a template function:

<code class="cpp">template<class x> struct A {
   template<int i> void f() {}
};

template<class t> void g()
{
   A<t> a;
   a.f();  // Compilation fails here
}</t></class></int></class></code>

The compiler fails to compile this code, reporting an error related to an invalid use of member and suggesting that '&' might have been forgotten.

Explanation

The error occurs because the code attempts to invoke a member template without explicitly specifying the 'template' keyword before it. According to the C Standard (14.2/4), when the name of a member template specialization is used after a dot or arrow in a postfix expression, or after a nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter, the member template name must be prefixed by the keyword 'template.' Otherwise, the name will be assumed to refer to a non-template.

Solution

To resolve this issue, the code must be modified to explicitly specify the 'template' keyword before the member template name:

<code class="cpp">template<class t> void g()
{
   A<t> a;
   a.template f();  // add 'template' keyword here
}</t></class></code>

With this modification, the compiler will be able to correctly identify and invoke the member template function, and the code will compile successfully.

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