Why Can't C Template Parameters Be Inferred for Class Constructors Like Function Templates?

Class Constructor Template Inference Controversy

Template parameters can be conveniently inferred from function arguments, as in the following code:

template <typename T>
void swap(T& a, T& b) {
  T temp = a;
  a = b;
  b = temp;
}

However, a similar approach is not allowed for class constructors, which has raised questions among C programmers. Why not?

The reason behind this inconsistency lies in the complexities of object construction. Constructors are not the only entry point for a class. Copy constructors and assignment operators also play vital roles, and inferring template parameters from only the constructor could lead to ambiguities.

Consider the following example:

template <typename T>
class Variable {
  T data;
public:
  Variable(T d) { data = d; }
};

If template inference were allowed, the following code would be valid:

Variable var(2); // Equivalent to Variable<int> var(2);

But what if we used copy construction or assignment operators?

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

In this case, it would be challenging for the compiler to determine the template type of MyClass pm.

To alleviate this issue, C 17 introduced type deduction from constructor arguments. This allows template parameters to be inferred from constructor parameters, eliminating the need for explicit type arguments in certain situations. For example:

std::pair p(2, 4.5); // Inferred as std::pair<int, double> p(2, 4.5);
std::tuple t(4, 3, 2.5); // Inferred as std::tuple<int, int, double> t(4, 3, 2.5);

It's important to note that this inference feature is still under development and may be subject to modifications in future C standards. Nevertheless, it marks a significant step towards simplifying code and improving type safety.

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