P1147R0
Printing volatile Pointers

1. Introduction

Printing pointers to volatile types with standard library output streams has unexpected results. Consider the following code:

#include <iostream>

int main()
{
           int* p0 = reinterpret_cast<         int*>(0xdeadbeef);
  volatile int* p1 = reinterpret_cast<volatile int*>(0xdeadbeef);

  std::cout << p0 << std::endl;
  std::cout << p1 << std::endl;
}

This produces the following output:

0xdeadbeef
1

What happened here? Well, basic_ostream has an operator<< for both const void* and bool:

27.7.5.1 Class template basic_ostream [ostream]

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_ostream : virtual public basic_ios<charT, traits> {
  public:
    // ...

    // [ostream.formatted], formatted output
    // ...
    basic_ostream<charT, traits>& operator<<(bool n);
    basic_ostream<charT, traits>& operator<<(const void* p);
    // ...

    // ...
  };
}

For std::cout << p0, p0 (an int*) is implicitly converted to const void* and operator<<(const void*) is called. However, for std::cout << p1, the operator<<(const void*) overload is not a match, as it discards qualifiers. Instead, the best match is operator<<(bool), so p1 (a volatile int*) is implicitly converted to bool and operator<<(bool) is called.

I suggest we add a new operator<<(const volatile void*) overload that const_casts away volatile and calls the operator<<(const void*) overload. Initially, I explored modifying the underlying <locale> methods and changing the existing operator<<(const void*) to operator<<(const volatile void*), however, early feedback from other committee members pushed me away from this direction, as there was concern that modifying <locale> would be an ABI breaking change.

Note that const_casting away volatile is safe here. We are not accessing or printing the value of the object that the pointer points to, we are just printing the value of the pointer itself.

2. Wording

NOTE: The � character is used to denote a placeholder section number which the editor shall determine.

Modify [ostream.inserters.arithmetic] as follows:

27.7.5.2.2 Arithmetic inserters [ostream.inserters.arithmetic]

operator<<(bool val);
operator<<(short val);
operator<<(unsigned short val);
operator<<(int val);
operator<<(unsigned int val);
operator<<(long val);
operator<<(unsigned long val);
operator<<(long long val);
operator<<(unsigned long long val);
operator<<(float val);
operator<<(double val);
operator<<(long double val);
operator<<(const void* val);

Effects: The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting. When val is of type bool, long, unsigned long, long long, unsigned long long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment:

bool failed = use_facet<
  num_put<charT, ostreambuf_iterator<charT, traits>>
    >(getloc()).put(*this, *this, fill(), val).failed();

basic_ostream<charT, traits>& operator<<(const volatile void* val);

Effects: Equivalent to return operator<<(const_cast<const void*>(val));

3. Acknowledgements

Thanks to JF Bastien, Marshall Clow, Billy O’Neal, and Louis Dionne for providing early review and feedback of this paper.