C++ Logo


Advanced search

[std-proposals] Type-safe printf and auto

From: Robin Rowe <robin.rowe_at_[hidden]>
Date: Sun, 12 Mar 2023 09:38:02 -0700
While type-safe iostream is the modern way, we still have the not so
type-safe printf. Modern compilers help by warning when encountering a
type mismatch on printf, such as:

 int i = 1;
 printf("Value of i is %u",i);// Warning: use %d

Anyone who does C/C++ code refactoring is aware of the side effect when
correcting the type of a variable, that it causes printf args mismatches
due to the type change:

 size_t n = 5;
 int i = 1;// Wrong, causes warning below...
 if(i > n)... ;// Warning: signed/unsigned mismatch...
 printf("Value of i is %d",i);
 size_t n = 5;
 size_t i = 1; // Fixed, yay, but breaks printf...
 if(i > n)... ;
 printf("Value of i is %d",i);// Warning: use %zu

Changing variable i to be the correct type breaks code using i with
printf. Fixing one warning causes new warnings everywhere the variable
is used with printf. In most cases, we simply want to print the value of
the variable. Don't really care what type it is. Note that when the
compiler outputs the printf type mismatch warning, it knows everything
about variable i, including its type and name.

Moreover, what about using printf with auto variables? It is necessary
we know the type of auto, but we specified auto intending to avoid that.

Wish we could do this:

 auto i = sizeof(int);
 printf("Value is %?, name is %??, type is %???",i,i,i);
   output: Value is 4, name is i, type is unsigned

Furthermore, in C++ code it could be nice if printf gave the same output
as cout for user-defined types:

 Point point(0,0,0);// My class Point with operator<<
 cout << point << endl;

    output: (0,0,0)

Thoughts? Suggestions?

To propose above as standard, touching both C and C++, what would be the


Received on 2023-03-12 16:38:08