Can Template Deduction Infer Type Arguments Based Solely on Function Return Type in C ?

Template Deduction for Function Based on Its Return Type: An Exploration

Introduction

In C , template deduction allows the compiler to infer the type arguments when invoking a templated function based on the actual arguments provided. This can simplify code by removing the need to explicitly specify type arguments.

The Proposed Scenario

The given problem presents a scenario where the user desires to use template deduction to allocate memory for objects of different types using a generic Allocate function. Specifically, they wish to achieve the following code:

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

Instead of the current implementation:

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

Templating the Allocate Function

The current Allocate function template is defined as follows:

<code class="cpp">template <typename T>
static GCPtr<T> Allocate();</code>

However, it's not possible to achieve the desired template deduction using solely the return type. The type deduction process primarily relies on the function's arguments.

A Workaround with Helper Function

To overcome this limitation, a helper function can be employed to hide the explicit type argument as shown below:

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

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

Alternative Syntax with C 11

C 11 introduces a syntax that allows discarding the explicit type declaration during template deduction:

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

Conclusion

While it's not possible to solely rely on template deduction via the return type, using a helper function or the C 11 syntax can provide a convenient alternative that aligns with the desired behavior. These techniques enable the allocation of different types of objects with simplified syntax, reducing the need for explicit type arguments.

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