Why does `std::forward` disable template argument deduction?

Disabling Template Argument Deduction with std::forward

Why is Template Argument Deduction Disabled with std::forward?

The std::forward function, in certain implementations (e.g., VS2010), employs a template metafunction called identity to disallow template argument deduction. This is done to maintain precise control over the behavior of std::forward in situations involving lvalue references and const lvalue references.

Implications of Using Template Argument Deduction

If std::forward were to utilize template argument deduction, it would result in inferring the template parameter (T) based on the input argument's type. This behavior becomes problematic when the input argument is an lvalue or const lvalue (e.g., variables with names). In such cases, template argument deduction would mistakenly deduce T as an lvalue reference or const lvalue reference, leading to incorrect type casting.

Consider the following example:

<code class="cpp">template<typename T>
T&& forward_with_deduction(T&& obj)
{
    return static_cast<T&&>(obj);
}</code>

When passing named variables or constants to forward_with_deduction, it would incorrectly interpret them as lvalue references or const lvalue references, resulting in T being deduced as such. Consequently, the static_cast operation would collapse the type to an lvalue reference or const lvalue reference, despite the expectation of a universal reference (T&&).

Example:

<code class="cpp">int x;
int&& z = std::move(x);

// Incorrectly deduces T to be int& and casts z as int&
forward_with_deduction(z); // calls test(int&) instead of test(int&&)</code>

Resolution in std::forward

To ensure proper casting of arguments in all cases, std::forward employs the identity metafunction. identity acts as a placeholding type that prevents the compiler from deducing the template parameter. As a result, std::forward always interprets the input argument as a universal reference, regardless of its actual type.

By casting the input argument to a universal reference, std::forward enables perfect forwarding of objects, even when they are lvalues or const lvalues. This behavior ensures that the type of the forwarded argument remains a universal reference throughout the forwarding process.

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