expand file names that have environment variables in their path

Question

What\'s the best way to expand

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

or

%MyPath%/filename.txt to c:\\Document         


        

Answer 1

Simple and portable:

#include 
#include 

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 *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