Other operators

Explanation

The function call operator provides function semantics for any object.

The ternary conditional operator checks the boolean value of the first expression and, depending on the resulting value, evaluates and returns either the second or the third expression.

Built-in function call operator

A function call expression, such as E(A1, A2, A3), consists of an expression that names the function, E, followed by a possibly empty list of expressions A1, A2, A3, ..., in parentheses.

The expression that names the function can be.

The function (or member) name specified by E can be overloaded, overload resolution rules used to decide which overload is to be called.

If E specifies a member function, it may be virtual, in which case the final overrider of that function will be called, using dynamic dispatch at runtime.

To call the function, all expressions A1, A2, A3, etc, provided as arguments are evaluated in arbitrary order, and each function parameter is initialized with its corresponding argument after implicit conversion if neccessary. If the call is made to a member function, then the this pointer to current object is converted as if by explicit cast to the this pointer expected by the function. The initialization and destruction of each parameter occurs in the context of the caller, which means, for example, that if constructor of a parameter throws an exception, the exception handlers defined within the function, even as a function-try block, are not considered. If the function is a variadic function, default argument promotions are applied to all arguments matched by the ellipsis parameter.

The return type of a function call expression is the return type of the chosen function, decided using static binding (ignoring the virtual keyword), even if the overriding function that's actually called returns a different type. This allows the overriding functions to return pointers or references to classes that are derived from the return type returned by the base function, i.e. C++ supports covariant return types. If E specifies a destructor, the return type is void.

The value category of a function call expression is lvalue if the function returns an lvalue reference or an rvalue reference to function, is an xvalue if the function returns an rvalue reference to object, and is a prvalue otherwise. If the function call expression is a prvalue of object type, it must have complete type except when used as an operand to decltype.

Function call expression is similar in syntax to value initialization T(), to function-style cast expression T(A1), and to direct initialization of a temporary T(A1, A2, A3, ...), where T is the name of a type.

#include <cstdio>
struct S
{
    int f1(double d) {
        printf("%f \n", d); // variable argument function call
    }
    int f2() {
        f1(7); // member function call, same as this->f1()
               // integer argument converted to double
    }
};
void f() {
   puts("function called"); // function call
}
int main()
{
    f(); // function call
    S s;
    s.f2(); // member function call
}

function called
7.000000

Built-in comma operator

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded, and its side effects are completed before evaluation of the expression E2 begins (note that a user-defined operator, cannot guarantee sequencing).

The type, value, and value category of the result of the comma expression are exactly the type, value, and value category of the second operand, E2. If E2 is a temporary, the result of the expression is that temporary. If E2 is a bit-field, the result is a bit-field.

The comma in various comma-separated lists, such as function argument lists (f(a, b, c)) and initializer lists int a[] = {1,2,3}, is not the comma operator. If the comma operator needs to be used in such contexts, it has to be parenthesized: f(a, (n++, n+b), c).

#include <iostream>
int main()
{
    int n = 1;
    int m = (++n, std::cout << "n = " << n << '\n', ++n, 2*n);
    std::cout << "m = " << (++m, m) << '\n';
}

n = 2
m = 7

Conditional operator

The first operand of the conditional operator is evaluated and contextually converted to bool. After both the value evaluation and all side effects of the first operand are completed, if the result was true, the second operand is evaluated. If the result was false, the third operand is evaluated.

The type and value category of the conditional expression E1 ? E2 : E3 are determined according to the following rules:

1. If either E2 or E3 has type void, one of the following must be true, or the program is ill-formed:
   • Either E2 or E3 (but not both) is a (possibly parenthesized) throw-expression. The result of the conditional operator has the type and the value category of the other expression. If the other expression is a bit field, the result is a bit field. Such conditional operator is commonly used in C++11 constexpr programming.
   • Both E2 and E3 are of type void (including the case when they are both throw-expressions). The result is a prvalue of type void.
2. Otherwise, if E2 and E3 have different types, at least one of which is a (possibly cv-qualified) class type, or both are glvalues of the same value category and have the same type except for cv-qualification, then an attempt is made to convert one of the operands to match the type of the other, as described below. If both operands can be converted, or if only one can be converted but the conversion is ambiguous, the program is ill-formed. If no conversion can be performed, the operands are left unchanged. If only one operand can be converted, the conversion is applied and the converted operand is used in place of the original operand. In both cases, further analysis is then performed starting from step 3.

   An operand (call it S) of type TS can be converted to match the other operand (call it T) of type TT as follows:

   • If T is an lvalue, an implicit conversion to the type TT& is attempted, provided that the reference must bind directly for the conversion to succeed;
   • If T is an xvalue, an implicit conversion to the type TT&& is attempted, provided that the reference must bind directly for the conversion to succeed;
   • If T is an prvalue, or if neither the above conversions can be done and at least one of TS and TT is a (possibly cv-qualified) class type:
     • If both S and T have class type, and the underlying class types are the same class or one is a base class of another, then S can be converted to match T if the class of TT is the same type as, or a base class of, the class of TS and the cv-qualification of TT is the same as or greater than that of TS. The conversion changes S into a prvalue of type TT by by copy-initializing a temporary object of the base type.
     • Otherwise, an implicit conversion is attempted to the type T would have after applying the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions.
3. If E2 and E3 are glvalues of the same type and the same value category, then the result has the same type and value category, and is a bit-field if at least one of E2 and E3 is a bit-field.
4. Otherwise, the result is a prvalue. If E2 and E3 do not have the same type, and either has (possibly cv-qualified) class type, overload resolution is performed using the built-in candidates below to attempt to convert the operands to built-in types. If the overload resolution fails, the program is ill-formed. Otherwise, the selected conversions are applied and the converted operands are used in place of the original operands for step 5.
5. The lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions are applied to the second and third operands. Then,
   • If both E2 and E3 now have the same type, the result is of that type. If that type is a class type, the result is a prvalue temporary of that type, copy-initialized from whatever operand was selected after evaluating E1.
   • If both E2 and E3 have arithmetic or enumeration type: the usual arithmetic conversions are applied to bring them to common type, and that type is the result.
   • If both E2 and E3 are pointers, or one is a pointer and the other is a null pointer constant, then pointer conversions and qualification conversions are applied to bring them to common type, and that type is the result.
   • If both E2 and E3 are pointers to members, or one is a pointer to member and the other is a null pointer constant, then pointer-to-member conversions and qualification conversions are applied to bring them to common type, and that type is the result.
   • If both E2 and E3 are null pointer constants, and at least one of which is of type std::nullptr_t, then the result's type is std::nullptr_t.
   • In all other cases, the program is ill-formed.

For every pair of promoted arithmetic types L and R and for every type P, where P is a pointer, pointer-to-member, or scoped enumeration type, the following function signatures participate in the overload resolution performed in step 4 of the rules above:

LR operator?:(bool, L, R );

P operator?:(bool, P, P );

where LR is the result of usual arithmetic conversions performed on L and R. The operator “?:” cannot be overloaded, these function signatures only exist for the purpose of overload resolution.

The return type of a conditional operator is also accessible as the binary type trait std::common_type.

#include <string>
#include <stdexcept>
struct Node
{
    Node* next;
    int data;
    // deep-copying copy constructor
    Node(const Node& other)
      : next(other.next ? new Node(*other.next) : NULL)
      , data(other.data)
    {}
    Node(int d) : next(NULL), data(d) {}
    ~Node() { delete next ; }
};
int main()
{   
    // simple rvalue example
    int n = 1>2 ? 10 : 11;  // 1>2 is false, so n = 11
    // simple lvalue example
    int m = 10; 
    (n == m ? n : m) = 7; // n == m is false, so m = 7
    // throw example
    std::string str = 2+2==4 ? "ok" : throw std::logic_error("2+2 != 4");
}

Standard library

Many classes in the standard library overload operator() to be used as function objects.

The comma operator is not overloaded by any class in the standard library. The boost library uses operator, in boost.assign, boost.spirit, and other libraries. The database access library SOCI also overloads operator,.

See also

Operator precedence.

Operator overloading.