[Bug libstdc++/39416] New: std::map::operator[] inserts a new item in RHS context

["glyn at adgie dot f9 dot co dot uk" <gcc-bugzilla at gcc dot gnu dot org>]

#include <iostream>
#include <map>
#include <string>

// Demonstrate unexpected behaviour of std::map::operator[] when used in a read
// context, e.g. on the right hand side of an assignment.
//
// The intended application is a shell environment table, where accessing a
// non-existent variable should yield an empty string, but not alter the table.
// In my shell, I sometimes wish to distinguish between an existing variable
// with an empty value, and a non-existent variable. The behaviour of
// std::map::operator[] defeats this, so I have to use the more verbose
// std::map::find interface for reading from the map.

int main(int argc, char * argv[])
{
        typedef std::map<std::string, std::string> env_type;

        env_type env;
        env["my_var"] = "Hello world!"; // Add an item.

        std::cout << env["thing"] << std::endl; // Empty string, as expected.

        // List all variables and their values.
        for(env_type::const_iterator it = env.begin(); it != env.end(); ++it)
        {
                std::cout << it->first << '=' << it->second << std::endl;
        }

        // Oh no! The non-existent variable 'thing' has been added to the map.
        //
        // my_var=Hello world!
        // thing=

        return 0;
}


-- 
           Summary: std::map::operator[] inserts a new item in RHS context
           Product: gcc
           Version: 4.3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: glyn at adgie dot f9 dot co dot uk


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39416