C++ constructor initializer list

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C++ constructor initializer list

Initalizer list example
Initializer list usage
Initializer list order

1. Initalizer list example

Compare two versions of the Rectangle struct constructors:

Constructors not using the initalizer lists:


struct Rectangle {
    int width;  // member variable
    int height; // member variable
    char* ptr_name;
    Rectangle()
    {
        width = 1;
        height = 1;
        ptr_name = nullptr;
        set_name( "UNKNOWN" );
    }
    Rectangle( char const* ptr_name_, int width_  )
    {
        width = width_;
        height = width_;
        ptr_name = nullptr;
        set_name( ptr_name_ );
    }
    Rectangle( char const* ptr_name_, int width_ , int height_ )
    {
        width = width_;
        height = height_;
        ptr_name = nullptr;
        set_name( ptr_name_ );
    }
    //...
};//struct Rectangle

Constructors using the initalizer lists:


struct Rectangle {
    int width;  // member variable
    int height; // member variable
    char* ptr_name;
    Rectangle()
        :
        width( 1 ),
        height( 1 ),
        ptr_name( nullptr )
    {
        set_name( "UNKNOWN" );
    }
    Rectangle( char const* ptr_name_, int width_  )
        :
        width( width_ ),
        height( width_ ),
        ptr_name( nullptr )
    {
        set_name( ptr_name_ );
    }
    Rectangle( char const* ptr_name_, int width_ , int height_ )
        :
        width( width_ ),
        height( height_ ),
        ptr_name( nullptr )
    {
        set_name( ptr_name_ );
    }
    //...
};//struct Rectangle

2. Initializer list usage

Initializer list is a preferred constructor syntax by most professionals for member initialization.
Initializer list must be used for:
- References
- Data members without default constructors
- const member variables
If you do not explicitly initialize a data member in the initializer list, the compiler automatically initializes it using member's default constructor. For objects without default constructor this will be an error.

3. Initializer list order

Initializer list order should follow the order of data member declarations in your struct or class

Because constructor call order is very important, modern compilers will generate errors when initializer list does not follow the order of data members. For exaample,


struct Rectangle {
    int width;  // member variable
    int height; // member variable
    char* ptr_name;

    Rectangle()
        :
        ptr_name( nullptr ), // out of order
        height( 1 ), // out od order
        width( 1 ) // out of order
    {
        set_name( "UNKNOWN" );
    }

    //...
};//struct Rectangle

When compiled with GNU GCC compiler, the above code will give you warning messages like this:


 In constructor 'Rectangle::Rectangle()':
7:11: warning: 'Rectangle::ptr_name' will be initialized after [-Wreorder]
6:9: warning:   'int Rectangle::height' [-Wreorder]
9:5: warning:   when initialized here [-Wreorder]
6:9: warning: 'Rectangle::height' will be initialized after [-Wreorder]
5:9: warning:   'int Rectangle::width' [-Wreorder]
9:5: warning:   when initialized here [-Wreorder]