Why Does `std::make_pair` Fail with Explicit Template Arguments in C 11?

Unraveling the Enigma: std::make_pair Conundrum with Explicit Template Parameters

In the realm of C , the std::make_pair function has risen to prominence as a convenient tool for creating pairs. However, a peculiar issue arises when attempting to employ this function with explicitly specified template parameters in C 11. This article delves into the enigmatic behavior behind this situation.

Error at Hand

Consider the following code snippet that attempts to create a pair of a string and an integer with explicit template arguments:

std::pair<std::string, int>& b = std::make_pair<std::string, int>(s, 7);

Instead of the expected compilation success, a cryptic error message emerges:

error: no matching function for call to 'make_pair(std::string&, int)'

Unveiling the Mystery

The source of this error lies in the incorrect usage of std::make_pair. The function was designed to infer the template parameters based on the provided arguments. Explicitly stating these parameters hinders this inference process.

Under the Hood

The C 11 implementation of std::make_pair takes two parameters of type T&& and U&&, where T and U are template type parameters. When explicitly specifying the template arguments, as seen in the problematic code, no argument deduction occurs. Instead, the specified types are directly substituted into the template declaration, resulting in the following:

make_pair(std::string&& argT, int&& argU);

The Crucial Role of Rvalue References

Note that both parameters in the explicitly specified template are rvalue references (&&), which can only bind to rvalue expressions. In the provided code, s is an lvalue (not a temporary and not being moved). Consequently, the function template fails to match the arguments, leading to the compilation error.

Why It Works Without Explicit Arguments

When omitting the explicit template arguments, argument deduction takes place. Due to the special nature of rvalue reference parameters in templates (reference collapsing), an rvalue reference parameter of type A&&, where A is a template type parameter, can bind to any type of A. Whether A is an lvalue, rvalue, qualified or not, an A&& can bind to it.

In the example code, s is an lvalue of type std::string and 7 is an rvalue of type int. The compiler deduces T to be std::string& and U to be int, allowing s and 7 to bind successfully to the inferred parameter types.

Conclusion

To avoid such errors, adhere to the following principle: If a template argument can be deduced from the function arguments, allow the compiler to perform the deduction. Explicitly providing arguments is often unnecessary and can lead to unexpected results. By harnessing the power of argument deduction, programmers can simplify their code and enhance its readability while maintaining its correctness.

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