construction order of object's members

[nuderobmonkey]

In which order will the elements of an object get constructed?

E.g., if we have a class with some members and a user-defined constructor like this:

class FrameRate : private sf::NonCopyable
{
private:
    sf::Text mText;
    sf::Font mFont;
    sf::RenderWindow mWindow;
    sf::Clock mClock;
    sf::Time mTime;
    sf::String mString;
private:
    void update();
    void render();
public:
    FrameRate();
    void run();
};

FrameRate::FrameRate()
: mWindow( sf::VideoMode(800,200), "Framerate" )
, mText()
, mClock()

{
    mFont.loadFromFile("/usr/share/fonts/truetype/ubuntu/UbuntuMono-B.ttf" );
    mString = sf::String("framerate: ");
    mText.setFont(mFont);
    mText.setCharacterSize(100);
    mText.setFillColor(sf::Color(0xAAAAAA));
    mText.setString(mString);
}

[Peter87]

Members are initialized in the order they are declared. So mText will be initialized first, then mFont, mWindow, mClock, mTime, mString, in that order.

[nuderobmonkey]

Thank you Peter87!

[nuderobmonkey]

I wrote a program for testing what Peter87 said:

#include <iostream>
using std::cerr;

struct A { 
    A(){ cerr << "A()\n"; }
    A(A& a){ cerr << "A(A& a)\n"; }
    ~A(){ cerr << "~A()\n"; }
};
struct B { 
    B(){ cerr << "B()\n"; } 
    ~B(){ cerr << "~B()\n"; } 
};
struct C { 
    C(){ cerr << "C()\n"; } 
    ~C(){ cerr << "~C()\n"; }
};

struct Test {
    // This order destines in which order the objects will get constructed
    // and destructed (destruction order is their reverse order of
    // the construction order).
    A objAA;
    A objA;
    B objB;
    C objC;
    // This order doesn't influence the construction/destruction order.
    Test() : objC(), objB(), objAA(objA), objA(){}
};

int main ()
{
    Test testobj;
}

https://rextester.com/ZYVD92791
output:

A(A& a)
A()
B()
C()
~C()
~B()
~A()
~A()

But this program threw to me the question: Why can at line 22 objAA get constructed although it depends on objA at line 23?

[nuderobmonkey]

Sorry for my last question, at the online compiler were the compiler warnings (per default) disabled. Enabling them throws the warning that objA will be passed uninitialized to objA.

[Peter87]

Why can at line 22 objAA get constructed although it depends on objA at line 23?

A good compiler will warn you about this. If you're using GCC or Clang you should at least use the -Wall compiler flag. https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wall

This is what GCC says:

test.cpp: In constructor ‘Test::Test()’:
test.cpp:25:7: warning: ‘Test::objC’ will be initialized after [-Wreorder]
     C objC;
       ^~~~
test.cpp:24:7: warning:   ‘B Test::objB’ [-Wreorder]
     B objB;
       ^~~~
test.cpp:27:5: warning:   when initialized here [-Wreorder]
     Test() : objC(), objB(), objAA(objA), objA(){}
     ^~~~
test.cpp:24:7: warning: ‘Test::objB’ will be initialized after [-Wreorder]
     B objB;
       ^~~~
test.cpp:22:7: warning:   ‘A Test::objAA’ [-Wreorder]
     A objAA;
       ^~~~~
test.cpp:27:5: warning:   when initialized here [-Wreorder]
     Test() : objC(), objB(), objAA(objA), objA(){}
     ^~~~