C++ Class Composition: Constructor? or ()operator? -

i writing code book exercise , ran simple question. best if show examples first.

• first class: fclass heaader

  class fclass { public:     explicit fclass( int = 0, int = 0 );     fclass& operator()( int, int );     void print(); private:     int x;     int y; } 

• first class: fclass cpp

  fclass::fclass( int a, int b ) : x(a), y(b) { } fclass& fclass::operator()( int a, int b ) {     x = a;     y = b; } void fclass::print() {     cout << "x: " << x << "\ny: " << y << endl; } 

• second class, sclass.h

  class sclass { public:     explicit sclass( int = 0, int = 0 );     void print(); private:     fclass firstclass; } 

• second class, sclass.cpp

  sclass::sclass( int a, int b ) : firstclass( a, b ) { } void sclass::print() {     firstclass.print(); } 

• the main function

  int main() {     sclass secondclass( 1, 2 );     secondclass.print(); } 

now question when deleted operator() function in fclass. code still worked! understand 'explicit' declaration of fclass's constructor should prohibit sclass's fclass definition in constructor (firstclass( a, b )) because fclass explicitly defined in private member declaration. therefore, re-initialize in sclass firstclass( a, b ), operator function () should defined (as did). why code work without operator() definition?

to add more comments on question, understanding, statement 'firstclass( a, b )' should not call fclass's explicit constructor, because not first time initialized. again, understanding, constructors called when class first initialized construct class. firstclass initialized , constructed in header file...

you confused explicit keyword , operator() functions.

the keyword explicit prevents constructor getting called construct object implicitly. simple example:

let's there function:

void foo(fclass obj) {...} 

without explicit keyword, able call function using syntax:

foo({1, 2}); 

the arugment {1,2} used call constructor , resulting object passed foo.

with explicit keyword, won't able use that. you'll have use:

foo(fclass{1, 2}); 

or

foo(fclass(1, 2)); 

the operator() function has no relationship explicit keyword , construction of objects. means, can use:

fclass obj(1, 2); obj(10, 20); 

you said:

as understand explicit declaration of fclass's constructor should prohibit sclass's fclass definition in constructor (firstclass( a, b )) because fclass explicitly defined in private member declaration.

your understanding not correct.

the line

fclass firstclass; 

in definition of class sclass says class has member variable named firstclass , type fclass. doesn't set value of variable when object of sclass constructed.

in definition of constructor of sclass, initializing member using : firstclass( a, b ), right thing do. if skipped it, member initialized if used :firstclass(), valid way initialize object of type fclass.