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

Dec 08, 2024 pm 05:44 PM

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:

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template <typename T>

void swap(T& a, T& b) {

  T temp = a;

  a = b;

  b = temp;

}

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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:

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template <typename T>

class Variable {

  T data;

public:

  Variable(T d) { data = d; }

};

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

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

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But what if we used copy construction or assignment operators?

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MyClass m(string s);

MyClass *pm;

*pm = m;

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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:

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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);

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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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