Give 'std::string' a non-const '.data()' member function.

Document number: P0272R1
Date: 2016-03-04
Reply-to: David Sankel
Audience: Library Evolution

Abstract

Is std::string's lack of a non-const .data() member function an oversight or an intentional design based on pre-C++11 std::string semantics? In either case, this lack of functionality tempts developers to use unsafe alternatives in several legitimate scenarios. This paper argues for the addition of a non-const .data() member function for std::string to improve uniformity in the standard library and to help C++ developers write correct code.

Changes

P0272R1 added a node for Annex C.4 wrt. the implications of this change and fixed minor formatting issues.

Introduction

This paper brings to discussion an issue that was originally brought forward in LWG issue 2391 by Michael Bradshaw. It was moved to a LEWG issue in 2015 and hasn't been looked at since. This is being elevated to a paper in the hopes that it will finally be addressed.

Use Cases

C libraries occasionally include routines that have char * parameters. One example is the lpCommandLine parameter of the CreateProcess function in the Windows API. Because the data() member of std::string is const, it cannot be used to make std::string objects work with the lpCommandLine parameter. Developers are tempted to use .front() instead, as in the following example.

std::string programName;

// ...

if( CreateProcess( NULL, &programName.front(), /* etc. */ ) ) {
  // etc.
} else {
  // handle error
}

Note that when programName is empty, the programName.front() expression causes undefined behavior. A temporary empty C-string fixes the bug.

std::string programName;

// ...

if( !programName.empty() ) {

  char emptyString[] = {'\0'};

  if( CreateProcess( NULL, programName.empty() ? emptyString : &programName.front(), /* etc. */ ) ) {
    // etc.
  } else {
    // handle error
  }
}

If there were a non-const .data() member, as there is with std::vector, the correct code would be straightforward.

std::string programName;

// ...

if( !programName.empty() ) {

  char emptyString[] = {'\0'};

  if( CreateProcess( NULL, programName.data(), /* etc. */ ) ) {
    // etc.
  } else {
    // handle error
  }
}

A non-const .data() std::string member function is also convenient when calling a C-library function that doesn't have const qualification on its C-string parameters. This is common in older codes and those that need to be portable with older C compilers.

Wording

The wording here was taken directly from LWG issue 2391.

Change class template basic_string synopsis, [basic.string], as indicated:

  namespace std {
    template<class charT, class traits = char_traits<charT>,
    class Allocator = allocator<charT> >
    class basic_string {
    public:
      […]
      // 21.4.7, string operations:
      const charT* c_str() const noexcept;
      const charT* data() const noexcept;
      charT* data() noexcept;
      allocator_type get_allocator() const noexcept;
      […]
    };
  }