c++ - Default constructor missing - but I'm not calling it?

Question

I'm writing a C++ application in which I have a Controller class with two nested structs, defined in my header file as follows:

class Controller {
    struct help_message {   // controller.hpp, line 19
        std::string summary;
        std::string details;
        help_message(const std::string&, const std::string&);
    };

    struct player_command {
        cmd_t cmd;
        help_message help;
        // cmd_t is my own typedef, irrelevant for this question
        player_command(const cmd_t&, const help_message&);
    };

    // more members...
};

In my source file, I have this:

Controller::player_command::player_command(const Controller::cmd_t& c, const help_message& h) {
    cmd = c;
    help = h;
};

Controller::help_message::help_message(const std::string& s, const std::string& d) {
    summary = s;
    details = d;
};

which I thought was fine, but when I compile, this is what I get (controller.cpp line 12 is the first line of the source snippet above):

g++  -g -Wall -std=c++0x  -c -o controller.o controller.cpp
controller.cpp: In constructor ‘palla::Controller::player_command::player_command(void (palla::Controller::* const&)(const args_t&), const palla::Controller::help_message&)’:
controller.cpp:12:93: error: no matching function for call to ‘palla::Controller::help_message::help_message()’
controller.cpp:12:93: note: candidates are:
In file included from controller.cpp:7:0:
controller.hpp:22:3: note: palla::Controller::help_message::help_message(const string&, const string&)
controller.hpp:22:3: note:   candidate expects 2 arguments, 0 provided
controller.hpp:19:9: note: palla::Controller::help_message::help_message(const palla::Controller::help_message&)
controller.hpp:19:9: note:   candidate expects 1 argument, 0 provided
controller.hpp:19:9: note: palla::Controller::help_message::help_message(palla::Controller::help_message&&)
controller.hpp:19:9: note:   candidate expects 1 argument, 0 provided
make: *** [controller.o] Error 1

From what I can deduce, the compiler is somewhere trying to call the default constructor of help_message, which doesn't exist. It then tries to match the call up with the constructor I created as well as with the generated copy-constructor and assignment operator, and failing each on the number of arguments.

But what part of my code is calling a default constructor? And how do I fix this error?

Answer 1

The player_command() constructor first default constructs help and then assigns to it:

Controller::player_command::player_command(const Controller::cmd_t& c, const help_message& h) {
    cmd = c;
    help = h;
};

Change that to:

Controller::player_command::player_command(const Controller::cmd_t& c, const help_message& h)
:  cmd(c),
   help(h)
{
};

See Benefits of Initialization lists