Why Does Explicitly Specifying Template Parameters in `std::make_pair` Cause Compilation Errors in C 11?

Inability of std::make_pair to Compile When Explicitly Specifying Template Parameters in C 11

In C 11, std::make_pair takes two arguments, references to rvalues (T&& and U&&). However, explicitly specifying template parameters during its invokation prevents argument deduction, resulting in the substitution of rvalue references within the function template declaration. This mismatch, where lvalue arguments cannot bind to rvalue references, causes compilation errors.

Root Cause of the Compilation Error

Attempting to use std::make_pair with explicitly provided template arguments (e.g., std::make_pair()) leads to a mismatch between the expected rvalue references and the actual lvalue argument (std::string).

The Role of Rvalue References in Templates

Rvalue reference parameters in templates possess the unique ability to bind to any instance of their template type parameter, regardless of whether it's an lvalue, rvalue, or even qualified.

Compiler Behavior with Omitted Template Parameters

When template arguments are omitted during std::make_pair invocation, template argument deduction takes place. The compiler deduces template types from the provided arguments, in this case, std::string and int. The rvalue reference parameters (T&& and U&&) are deduced to std::string& and int&&, respectively. The resulting template argument for T is collapsed, eliminating the extra reference, allowing binding to the lvalue argument (std::string).

Solution and Best Practice

To avoid such errors, refrain from explicitly specifying template arguments for std::make_pair unless necessary. Allow the compiler to perform argument deduction, which typically leads to the desired behavior. If an unexpected result occurs, a clear compilation error will help pinpoint the issue.

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