expand file names that have environment variables in their path

What's the best way to expand

${MyPath}/filename.txt to /home/user/filename.txt

or

%MyPath%/filename.txt to c:\Documents and settings\user\filename.txt

with out traversing the path string looking for environement variables directly? I see that wxWidgets has a wxExpandEnvVars function. I can't use wxWidgets in this case, so I was hoping to find a boost::filesystem equivalent or similar. I am only using the home directory as an example, I am looking for general purpose path expansion.

For UNIX (or at least POSIX) systems, have a look at wordexp:

#include <iostream>
#include <wordexp.h>
using namespace std;
int main() {
  wordexp_t p;
  char** w;
  wordexp( "$HOME/bin", &p, 0 );
  w = p.we_wordv;
  for (size_t i=0; i<p.we_wordc;i++ ) cout << w[i] << endl;
  wordfree( &p );
  return 0;
}

It seems it will even do glob-like expansions (which may or may not be useful for your particular situation).

On Windows, you can use ExpandEnvironmentStrings. Not sure about a Unix equivalent yet.

If you have the luxury of using C++11, then regular expressions are quite handy. I wrote a version for updating in place and a declarative version.

#include <string>
#include <regex>

// Update the input string.
void autoExpandEnvironmentVariables( std::string & text ) {
    static std::regex env( "\\$\\{([^}]+)\\}" );
    std::smatch match;
    while ( std::regex_search( text, match, env ) ) {
        const char * s = getenv( match[1].str().c_str() );
        const std::string var( s == NULL ? "" : s );
        text.replace( match[0].first, match[0].second, var );
    }
}

// Leave input alone and return new string.
std::string expandEnvironmentVariables( const std::string & input ) {
    std::string text = input;
    autoExpandEnvironmentVariables( text );
    return text;
}

An advantage of this approach is that it can be adapted easily to cope with syntactic variations and deal with wide strings too. (Compiled and tested using Clang on OS X with the flag -std=c++0x)

Simple and portable:

#include <cstdlib>
#include <string>

static std::string expand_environment_variables( const std::string &s ) {
    if( s.find( "${" ) == std::string::npos ) return s;

    std::string pre  = s.substr( 0, s.find( "${" ) );
    std::string post = s.substr( s.find( "${" ) + 2 );

    if( post.find( '}' ) == std::string::npos ) return s;

    std::string variable = post.substr( 0, post.find( '}' ) );
    std::string value    = "";

    post = post.substr( post.find( '}' ) + 1 );

    const char *v = getenv( variable.c_str() );
    if( v != NULL ) value = std::string( v );

    return expand_environment_variables( pre + value + post );
}

expand_environment_variables( "${HOME}/.myconfigfile" ); yields /home/joe/.myconfigfile