Why Can\'t I Explicitly Invoke Template Constructors in C Initializer Lists?

Explicit Invocation of Template Constructors in Initializer Lists

In C , it is not possible to explicitly invoke a template constructor within an initializer list. Consider the following example:

struct T { 
    template<class> T();
};

struct U {
    U() : t<void>() {} //does not work
    T t;
};

This code will fail to compile, as the C Standard explicitly disallows explicit template argument lists for constructor member function templates. This is due to the unique syntax of constructors, which do not have a function name.

Explanation: The C Standard states in Section 14.8.1/7:

"Because the explicit template argument list follows the function template name, and because conversion member function templates and constructor member function templates are called without using a function name, there is no way to provide an explicit template argument list for these function templates."

Workaround: If the explicit invocation of a template constructor is necessary within an initializer list, a workaround can be employed using a custom type with a constructor that accepts the template parameters as an argument:

struct T { 
    template<class U> T(identity<U>);
};

struct U {
    U() : t(identity<void>()) {}
    T t;
};

In this case, the identity type from the Boost library or the std::type_identity from C 20 can be used to represent the template parameter type. This allows the template constructor to be invoked indirectly using an object of this custom type.

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