Difference between revisions of "cpp/language/operator other"

Revision as of 23:16, 26 January 2022

Explanation

The function call operator provides function semantics for any object.

The conditional operator (colloquially referred to as ternary conditional) 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

The function call expressions have the form

where

• E is an expression that names a function
• A1, A2, A3,... is a possibly empty list of arbitrary expressions or braced-init-lists(since C++11), except the comma operator is not allowed at the top level to avoid ambiguity.

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,

Each function parameter is initialized with its corresponding argument after implicit conversion if necessary. If there is no corresponding argument, the corresponding default argument is used, and if there is none, the program is ill-formed. 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. It is implementation-defined whether the lifetime of a parameter ends when the function in which it is defined returns or at the end of the enclosing full-expression.

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 the prvalue is not materialized, such as(since C++17) when used as the operand of decltype (or as the right operand of a built-in comma operator expression that is the operand of 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)
    {
        return printf("%f \n", d); // variable argument function call
    }
 
    int f2()
    {
        return 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

The comma operator expressions have the form

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded (although if it has class type, it won't be destroyed until the end of the containing full expression), and its side effects are completed before evaluation of the expression E2 begins (note that a user-defined operator, cannot guarantee sequencing)(until C++17).

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 expression(since C++17), the result of the expression is that temporary expression(since C++17). 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 conditional operator expressions have the form

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:

std::string str = 2 + 2 == 4 ? "OK" : throw std::logic_error("2 + 2 != 4");

2 + 2 == 4 ? throw 123 : throw 456;

struct A {};
 
struct B : A {};
 
using T = const B;
 
A a = true ? A() : T(); // Y = A(), TY = A, X = T(), TX = const B. Target = const A

int* intPtr;
 
using Mixed = decltype(true ? nullptr : intPtr);
 
static_assert(std::is_same_v<Mixed, int*>); // nullptr becoming int*

struct A
{
    int* m_ptr;
} a;
 
int* A::* memPtr = &A::m_ptr; // memPtr is a pointer to member m_ptr of A
 
// memPtr makes nullptr as type of pointer to member m_ptr of A
static_assert(std::is_same_v<decltype(false ? memPtr : nullptr), int*A::*>);
 
// a.*memPtr is now just pointer to int and nullptr also becomes pointer to int
static_assert(std::is_same_v<decltype(false ? a.*memPtr : nullptr), int*>);

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 5 of the rules above:

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 <iostream>
 
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
 
    //output the result
    std::cout << "n = " << n << "\nm = " << m;
}

n = 11
m = 7

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,.

Defect reports

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

See also

Operator precedence

Operator overloading