Does Top-Level Const Impact Function Signatures in C ?

Top-Level Const Does Not Affect Function Signature

In C , a function's paramater list determines the function signature, regardless of whether the parameters are declared with top-level const. This means that two functions with the same parameter types, but different const qualifiers, will have the same function signature.

int f(int); // can modify parameter
int f(const int); // cannot modify parameter

This behavior may seem counterintuitive, as the const qualifier should logically affect how the function can update its parameters. However, the caller's perspective is what matters:

• When passing a const value or reference, the caller guarantees that the parameter will not be modified.
• When passing a non-const value or reference, the caller does not make any such guarantee.

Therefore, from the caller's perspective, the function signature is the same regardless of the const qualifier on the parameter. To provide different functionalities, an appropriate form of overloading must be used, such as:

void f(const int&); // pass by const reference
void f(int&); // pass by non-const reference

In some scenarios, it may be desirable to create a copy of the parameter to preserve the const-ness. This can be achieved by passing the parameter by value:

T f(const F&); // pass by const reference
T& f(F&); // pass by non-const reference

However, creating a copy of the parameter can introduce performance overhead or invalid memory references if the caller intended to pass a temporary object. Hence, C does not automatically create copies for top-level const parameters.

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