Constructors and member initializer lists

Constructor is a special non-static member function of a class that is used to initialize objects of its class type.

In the definition of a constructor of a class, member initializer list specifies the initializers for direct and virtual base subobjects and non-static data members. ( Not to be confused with std::initializer_list ).

Syntax

Constructors are declared using member function declarators of the following form:

Where class-name must name the current class (or current instantiation of a class template), or, when declared at namespace scope or in a friend declaration, it must be a qualified class name.

The only specifiers allowed in the decl-specifier-seq of a constructor declaration are friend, inline, explicit and constexpr (in particular, no return type is allowed). Note that cv- and ref-qualifiers are not allowed either; const and volatile semantics of an object under construction don't kick in until the most-derived constructor completes.

The body of a function definition of any constructor, before the opening brace of the compound statement, may include the member initializer list, whose syntax is the colon character :, followed by the comma-separated list of one or more member-initializers, each of which has the following syntax.

struct S {
    int n;
    S(int); // constructor declaration
    S() : n(7) {} // constructor definition.
                  // ": n(7)" is the initializer list
                  // ": n(7) {}" is the function body
};
S::S(int x) : n{x} {} // constructor definition. ": n{x}" is the initializer list
int main()
{
    S s; // calls S::S()
    S s2(10); // calls S::S(int)
}

Explanation

Constructors have no names and cannot be called directly. They are invoked when initialization takes place, and they are selected according to the rules of initialization. The constructors without explicit specifier are converting constructors. The constructors with a constexpr specifier make their type a LiteralType. Constructors that may be called without any argument are default constructors. Constructors that take another object of the same type as the argument are copy constructors and move constructors.

Before the compound statement that forms the function body of the constructor begins executing, initialization of all direct bases, virtual bases, and non-static data members is finished. Member initializer list is the place where non-default initialization of these objects can be specified. For members that cannot be default-initialized, such as members of reference and const-qualified types, member initializers must be specified. No initialization is performed for anonymous unions or variant members that do not have a member initializer.

The initializers where class-or-identifier names a virtual base class are ignored during execution of constructors of any class that is not the most derived class of the object that's being constructed.

Names that appear in expression-list or brace-init-list are evaluated in scope of the constructor:

class X {
    int a, b, i, j;
public:
    const int& r;
    X(int i)
      : r(a) // initializes X::r to refer to X::a
      , b{i} // initializes X::b to the value of the parameter i
      , i(i) // initializes X::i to the value of the parameter i
      , j(this->i) // initializes X::j to the value of X::i
    { }
};

Exceptions that are thrown from member initializers may be handled by function-try-block.

Member functions (including virtual member functions) can be called from member initializers, but the behavior is undefined if not all direct bases are initialized at that point.

For virtual calls (if the bases are initialized), the same rules apply as the rules for the virtual calls from constructors and destructors: virtual member functions behave as if the dynamic type of *this is the class that's being constructed (dynamic dispatch does not propagate down the inheritance hierarchy) and virtual calls (but not static calls) to pure virtual member functions are undefined behavior.

struct S {
    int n = 42;   // default member initializer
    S() : n(7) {} // will set n to 7, not 42
};

struct A {
    A() : v(42) { }  // Error
    const int& v;
};

class Foo {
public: 
  Foo(char x, int y) {}
  Foo(int y) : Foo('a', y) {} // Foo(int) delegates to Foo(char,int)
};

Initialization order

The order of member initializers in the list is irrelevant: the actual order of initialization is as follows:

(Note: if initialization order was controlled by the appearance in the member initializer lists of different constructors, then the destructor wouldn't be able to ensure that the order of destruction is the reverse of the order of construction).

Example

#include <fstream>
#include <string>
#include <mutex>
 
struct Base {
    int n;
};   
 
struct Class : public Base
{
    unsigned char x;
    unsigned char y;
    std::mutex m;
    std::lock_guard<std::mutex> lg;
    std::fstream f;
    std::string s;
 
    Class ( int x )
      : Base { 123 }, // initialize base class
        x ( x ),      // x (member) is initialized with x (parameter)
        y { 0 },      // y initialized to 0
        f{"test.cc", std::ios::app}, // this takes place after m and lg are initialized
        s(__func__),   //__func__ is available because init-list is a part of constructor
        lg ( m ),      // lg uses m, which is already initialized
        m{}            // m is initialized before lg even though it appears last here
    {}                // empty compound statement
 
    Class ( double a )
      : y ( a+1 ),
        x ( y ), // x will be initialized before y, its value here is indeterminate
        lg ( m )
    {} // base class constructor does not appear in the list, it is
       // default-initialized (not the same as if Base() were used, which is value-init)
 
    Class()
    try // function-try block begins before the function body, which includes init list
      : Class( 0.0 ) //delegate constructor
    {
        // ...
    }
    catch (...)
    {
        // exception occurred on initialization
    }
};
 
int main() {
    Class c;
    Class c1(1);
    Class c2(0.1);
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

References

• C++11 standard (ISO/IEC 14882:2011):
• 12.1 Constructors [class.ctor]
• 12.6.2 Initializing bases and members [class.base.init]
• C++98 standard (ISO/IEC 14882:1998):
• 12.1 Constructors [class.ctor]
• 12.6.2 Initializing bases and members [class.base.init]