Can Explicit Template Constructors Be Invoked Using Initializer Lists?

How To Invoke Explicit Template Constructors Using Initializer Lists

Question:

Can explicit template constructors be invoked using initializer lists, as seen in the following example?

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

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

Answer:

Unfortunately, it is not possible to explicitly invoke template constructors using initializer lists.

According to the C Standard at 14.8.1/7, "[...] 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."

This means that constructors, which lack their own names, cannot accept template arguments explicitly. In your case, the compiler interprets t as a base-class type, causing the code to fail.

To address this issue, you can employ a workaround:

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

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

Here, identity acts as a placeholder for the template parameter. Within C 20, you can use std::type_identity as identity type.

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