Why Doesn't C Template Parameter Inference Work with Constructors (Until C 17)?

Why Can't C Infer Template Parameters from Constructors?

C allows the compiler to infer template parameters from function parameters, enabling concise and type-safe code. However, this feature is not available for class constructors. Why is this the case?

In the example code, the compiler cannot infer the template parameter for Variable because the constructor is not the only entry point for the class. Copy constructors and the assignment operator provide alternative ways to create and modify objects.

Consider the following scenario:

MyClass m(string s);
MyClass *pm;
*pm = m;

In this case, the compiler would not know what template type is required for MyClass pm. While it's possible to infer the type from the argument passed to the constructor, it becomes uncertain when assignments are involved, making it difficult to determine the intended type.

Additionally, there could be instances where type inference is undesirable. For example, a class may have constructors that accept different types for different purposes. Inferring the template type could impose unintended constraints on the class interface.

C 17 Development

It's worth noting that C 17 is expected to introduce type deduction from constructor arguments. This will enable the following syntax:

std::pair p(2, 4.5);
std::tuple t(4, 3, 2.5);

However, it's crucial to remember that type inference is a convenience feature and may not always be suitable. Understanding the reasons behind its limitations helps developers write robust and maintainable C code.

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