how to initialize a pointer to a function inside a class that is used as value in a map? - c++

Question

I am writing a program whose behaviour is to be controlled by user-defined settings, provided through a plain ASCII configuration file including keywords and values.
Since not all keywords may be provided by the user, there are also "built-in" settings.
The configuration values provided for different keywords will affect variables (all part of settings class) that will, in turn, determine the program's behaviour.
To translate the configuration values to these variables, I want to use parsing functions - one per configuration keyword.
To achieve this, I use a map, with a configuration keyword as a key, and a param_struct object as a value.
param_struct contains the default value for the configuration keyword, the value read from configuration file (which may be empty), and a pointer to a parsing function.
As a first step, I am trying to initialize the default values, but I keep getting compilation errors. Is there anything am I doing fundamentally wrong here?
settings.cpp: In member function 'std::map<std::__cxx11::basic_string<char>, param_struct> settings::create_builtin_config()':
settings.cpp:84:96: error: no matching function for call to 'param_struct::param_struct(const char [6], const char [1], <unresolved overloaded function type>)'
84 | map<string, param_struct> builtin_config = {{"MY_CONFIG_KEYWORD", param_struct("XYZZY","", update_log)}}; //
| ^
settings.cpp:84:98: error: could not convert '{{"MY_CONFIG_KEYWORD", <expression error>}}' from '<brace-enclosed initializer list>' to 'std::map<std::__cxx11::basic_string<char>, param_struct>'
84 | map<string, param_struct> builtin_config = {{"MY_CONFIG_KEYWORD", param_struct("XYZZY","", update_log)}}; //
| ^
| |
| <brace-enclosed initializer list>
// settings.h file
typedef void (*param_setter) (string, string);
class param_struct {
string default_value; // built-in default configuration value
string loaded_value; // configuration value loaded from the
param_setter parsing_function;
// param_setter parsing_function; // pointer to the respective parsing/initialization function
public:
friend class settings;
param_struct();
param_struct(string, string, param_setter);
~param_struct();
};
class settings {
map<string,param_struct> config_params; // keywords and the respective values + parsing/initialization functions
/* lots of other declarations skipped */
map <string, param_struct> create_builtin_config();
public:
void update_log(string builtin_param, string user_param); // dummy function that does nothing, except writing a log entry. used to test config parameter parsing
friend class param_struct;
}
//settings.cpp file:
settings::settings() {
config_params = create_builtin_config();
}
map<string, param_struct> settings::create_builtin_config() {
map<string, param_struct> builtin_config = {{"MY_CONFIG_KEYWORD", param_struct("XYZZY","", update_log)}}; //
return builtin_config;
}
param_struct::param_struct(string defaultval, string loaded, param_setter parser_fn) {
default_value = defaultval;
loaded_value = loaded;
parsing_function = parser_fn;
}

Answer 1

update_log is a member function.
Your param_setter type is a free function.
They don't match.
A member function has an implicit first argument -- a pointer to the class -- and isn't compatibile with free function pointers.
The easy solution is to store a std::function<void(string,string)>, then pass in [this](string s1, string s2){ update_log(s1, s2); }. This also makes explicit that you are passing in a pointer to the object here; as you are storing it, you'll want to delete your move/copy construct/assignment operators, or you'll get very surprising results.
Another option is to make update_log a static member function, which is compatible with a free function of the same signature. You lose access to the state of your class object, however.