void CreateProcess(int, float);

int int_param = ...;
float float_param = ...;

CreateProcess(0, 0); //Use both defaults
CreateProcess(int_param, 0); //Use default float
CreateProcess(0, float_param); //Use default int
CreateProcess(int_param, float_param); //No defaults

CreateProcess( (... ? 0 : int_param), (... ? 0 : float_param) );

template<typename ...Args>
explicit child(Args&&...args);

int int_param = ...;
float float_param = ...;

child c; //Use both defaults
child c(int_param); //Use default float
child c(float_param); //Use default int
child c(int_param, float_param); //No defaults
child c(float_param, int_param); //Argument order is irrelevant

if (...) {
    if (...)
        child c;
    else
        child c(float_param);
} else {
    if (...)
        child c(int_param);
    else
        child c(int_param, float_param);
}