Can Template Deduction Be Based Solely on Return Type in C ?

Template Deduction for Function Return Type

In C , template deduction is a mechanism that infers the generic type arguments for a template function or class based on the supplied arguments. This feature allows for more concise and type-safe code.

Consider the following code snippet:

<code class="cpp">class GC {
public:
    template <typename T>
    static GCPtr<T> Allocate();
};</code>

Here, the Allocate function is a generic function that allocates memory for an object of type T. To use this function, the caller must explicitly specify the type argument, as shown below:

<code class="cpp">GCPtr<A> ptr1 = GC::Allocate<A>();
GCPtr<B> ptr2 = GC::Allocate<B>();</code>

The question arises whether it is possible to eliminate the explicit type arguments based on the return type of the Allocate function. Unfortunately, it is not possible in C to perform template deduction based solely on the return type.

However, there are alternative ways to achieve a similar effect. One common approach is to use a helper function that automatically deduces the type:

<code class="cpp">// Helper function
template <typename T>
void Allocate(GCPtr<T>& p) {
    p = GC::Allocate<T>();
}

int main() {
    GCPtr<A> p = 0;
    Allocate(p);
}</code>

With this approach, the caller can simply pass a reference to the GCPtr object, and the helper function will infer the type and call the appropriate Allocate function.

In C 11 and later versions, the auto keyword can be used to simplify the type deduction even further:

<code class="cpp">auto p = GC::Allocate<A>(); // p is of type GCPtr<A></code>

By leveraging helper functions or the auto keyword, it is possible to achieve the desired functionality while maintaining the benefits of template deduction.

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