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Initializer list

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( Not to be confused with std::initializer_list ) They are the part of a constructor which is responsible for member and ancestor initialization

Syntax

constructor_signature : member_constructor_calls { constructor_body }

Explanation

The initializer list is the place where initialization of the object should occur, there is where the constructors for base classes and members are called. Members are initialized in the same order as they are declared, not as they appear in the initializer list.

If a parameter in the constructor has the same name as one of the members, the ambiguity of that identifier being passed in a constructor call inside the intializer list is resolved choosing the parameter (and not the member).

Members or base classes not present in the list will be default constructed

Example

struct Class : public Base
{
    int x;
    int y;
 
    Class ( int x )
      : Base ( 123 ), // initialize base class
        x ( x ),      // x (member) is initialized with x (parameter)
        y ( 0 )       // y initialized to 0
    {}                // empty constructor body
 
    Class ( double a )
      : y ( a+1 ),
        x ( y ) // x will be initialized before y, this means that its value here is undefined
    {}          // No base class constructor in list, this is the same as calling Base()
 
    Class()
    try
      : Base ( 789 ),
        x ( 0 ),
        y ( 0 )
    {
        // no exception
    }
    catch (...)
    {
        // exception occurred on initialization
    }
};