C++ quick tips: Deleted free functions

I’m sure that we all know about the = delete keyword introduced in c++11 and its application to class member functions. I’m not sure though if everyone knows that = delete can be applied to free standing functions as well.

Free standing functions application

Main reason to have this feature is to limit the number of overloads and narrow down implicit conversion. std::cref and std::ref are a good example. cppreference for std::ref lists several overloads. The following are deleted:

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template< class T >
void ref( const T&& ) = delete;
...
template< class T >
void cref( const T&& ) = delete;

This is to prevent overload resolution to accept incovations like:

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auto rw = std::ref(123);

So, attempts to use std::ref or std::cref with rvalues is basically forbidden as it would result in undefined behaviour.

Exotic use case

= delete usage is not limited only to function overloads! There’s nothing preventing us from using it on any function.

Consider the following:

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int printf(const char * format, ...) = delete;

int main() {
    printf("hello world");
    return 0;
}

The above will result in build error:

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delete.cpp: In function ‘int main()’:
delete.cpp:6:11: error: use of deleted function ‘int printf(const char*, ...)’
    6 |     printf("hello world");
      |     ~~~~~~^~~~~~~~~~~~~~~

Following the = delete declaration, usage of printf has been effectively forbidden.

Let’s not forget though that this is a hack since printf is declared more than once and compiler will, rightly so, complain about this fact as well:

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delete.cpp:3:5: warning: deleted definition of ‘int printf(const char*, ...)’ is not first declaration
    3 | int printf(const char * format, ...) = delete;
      |     ^~~~~~
In file included from /usr/include/c++/13.1.1/cstdio:42,
                 from delete.cpp:1:
/usr/include/stdio.h:356:12: note: previous declaration of ‘int printf(const char*, ...)’
  356 | extern int printf (const char *__restrict __format, ...);