Explicit type conversion

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Converts between types using a combination of explicit and implicit conversions.

Syntax

( new-type ) expression (1)
new-type ( expression-list(optional) ) (2)
new-type { expression-list(optional) } (3) (since C++11)
template-name ( expression-list(optional) ) (4) (since C++17)
template-name { expression-list(optional) } (5) (since C++17)
auto ( expression ) (6) (since C++23)
auto { expression } (7) (since C++23)

Returns a value of type new-type.

Explanation

1) When the C-style cast expression is encountered, the compiler attempts to interpret it as the following cast expressions, in this order:
a) const_cast<new-type>(expression);
b) static_cast<new-type>(expression), with extensions: pointer or reference to a derived class is additionally allowed to be cast to pointer or reference to unambiguous base class (and vice versa) even if the base class is inaccessible (that is, this cast ignores the private inheritance specifier). Same applies to casting pointer to member to pointer to member of unambiguous non-virtual base;
c) static_cast (with extensions) followed by const_cast;
d) reinterpret_cast<new-type>(expression);
e) reinterpret_cast followed by const_cast.
The first choice that satisfies the requirements of the respective cast operator is selected, even if it cannot be compiled (see example). If the cast can be interpreted in more than one way as static_cast followed by a const_cast, it cannot be compiled.
In addition, C-style cast notation is allowed to cast from, to, and between pointers to incomplete class type. If both expression and new-type are pointers to incomplete class types, it's unspecified whether static_cast or reinterpret_cast gets selected.
2) The functional-style cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name, that is, cases such as unsigned int(expression) and int*(expression) are not valid), followed by a comma-separated list of expressions in parentheses.
  • If there is exactly one expression in parentheses, this cast expression is exactly equivalent to the corresponding C-style cast expression.
  • If there are more than one expression or braced-init-list (since C++11) in parentheses, new-type must be a class with a suitably declared constructor. This expression is a prvalue of type new-type designating a temporary (until C++17)whose result object is (since C++17) direct-initialized with expression-list.
  • If there's no expression in parentheses: if new-type names a non-array complete object type, this expression is an prvalue of type new-type, designating a temporary (until C++17)whose result object is (possibly with added cv-qualifiers) (since C++17) of that type. If new-type is an object type, the object is value-initialized. If new-type is (possibly cv-qualified) void, the expression is a void prvalue without a result object (since C++17).
3) A single-word type name followed by a braced-init-list is a prvalue of the specified type designating a temporary (until C++17)whose result object is (since C++17) direct-list-initialized with the specified braced-init-list. If new-type is (possibly cv-qualified) void, the expression is a void prvalue without a result object (since C++17). This is the only cast expression that can create an array prvalue. (until C++20)
4,5) Same as (2,3), except first performs class template argument deduction.
6,7) The auto specifier is replaced with the deduced type of the invented variable x declared with auto x(expression); (which is never interpreted as a function declaration) or auto x{expression}; respectively. The result is always a prvalue of an object type.

As with all cast expressions, the result is:

  • an lvalue if new-type is a reference type;
  • an rvalue otherwise.
(until C++11)
  • an lvalue if new-type is an lvalue reference type or an rvalue reference to function type;
  • an xvalue if new-type is an rvalue reference to object type;
  • a prvalue otherwise.
(since C++11)

Ambiguity Resolution

In the case of an ambiguity between a expression statement with a function-style cast expression as its leftmost subexpression and a declaration statement, the ambiguity is resolved by treating it as a declaration. This disambiguation is purely syntactic: it does not consider the meaning of names occurring in the statement other than whether they are type names: 

struct M {};
struct L { L(M&); };
 
M n;
void f()
{
    M(m);    // declaration, equivalent to M m;
    L(n);    // ill-formed declaration
    L(l)(m); // still a declaration
}

The ambiguity above can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. The resolution is also to consider any construct that could possibly be a declaration a declaration: 

struct S
{
    S(int);
};
 
void foo(double a)
{
    S w(int(a)); // function declaration: has a parameter `a` of type int
    S x(int());  // function declaration: has an unnamed parameter of type int
 
    // Ways to avoid ambiguity:
    S y((int(a))); // object declaration: extra pair of parentheses
    S y((int)a);   // object declaration: C-style cast
    S z = int(a);  // object declaration: no ambiguity for this syntax
}

An ambiguity can arise from the similarity between a function-style cast and a type-id. The resolution is that any construct that could possibly be a type-id in its syntactic context shall be considered a type-id: 

// `int()` and `int(unsigned(a))` can both be parsed as type-id:
// `int()`            represents a function returning int
//                    and taking no argument
// `int(unsigned(a))` represents a function returning int
//                    and taking an argument of type unsigned
void foo(signed char a)
{
    sizeof(int());            // type-id (ill-formed)
    sizeof(int(a));           // expression
    sizeof(int(unsigned(a))); // type-id (ill-formed)
 
    (int()) + 1;            // type-id (ill-formed)
    (int(a)) + 1;           // expression
    (int(unsigned(a))) + 1; // type-id (ill-formed)
}

Example

Run this code
#include <cassert>
#include <iostream>
 
double f = 3.14;
unsigned int n1 = (unsigned int)f; // C-style cast
unsigned int n2 = unsigned(f);     // function-style cast
 
class C1;
class C2;
C2* foo(C1* p)
{
    return (C2*)p; // casts incomplete type to incomplete type
}
 
void cpp23_decay_copy_demo()
{
    auto inc_print = [](int& x, const int& y)
    {
        ++x;
        std::cout << "x:" << x << ", y:" << y << '\n';
    };
 
    int p{1};
    inc_print(p, p); // prints x:2 y:2, because param y here is an alias of p
    int q{1};
    inc_print(q, auto{q}); // prints x:2 y:1, auto{q} (C++23) casts to prvalue,
                           // so the param y is a copy of q (not an alias of q)
}
 
// In this example, C-style cast is interpreted as static_cast
// even though it would work as reinterpret_cast
struct A {};
struct I1 : A {};
struct I2 : A {};
struct D : I1, I2 {};
 
int main()
{
    D* d = nullptr;
//  A* a = (A*)d;                   // compile-time error
    A* a = reinterpret_cast<A*>(d); // this compiles
    assert(a == nullptr);
 
    cpp23_decay_copy_demo();
}

Output: 

x:2 y:2
x:2 y:1

References

  • C++20 standard (ISO/IEC 14882:2020):
  • 7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]
  • 7.6.3 Explicit type conversion (cast notation) [expr.cast]
  • C++17 standard (ISO/IEC 14882:2017):
  • 8.2.3 Explicit type conversion (functional notation) [expr.type.conv]
  • 8.4 Explicit type conversion (cast notation) [expr.cast]
  • C++14 standard (ISO/IEC 14882:2014):
  • 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
  • 5.4 Explicit type conversion (cast notation) [expr.cast]
  • C++11 standard (ISO/IEC 14882:2011):
  • 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
  • 5.4 Explicit type conversion (cast notation) [expr.cast]
  • C++03 standard (ISO/IEC 14882:2003):
  • 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
  • 5.4 Explicit type conversion (cast notation) [expr.cast]
  • C++98 standard (ISO/IEC 14882:1998):
  • 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
  • 5.4 Explicit type conversion (cast notation) [expr.cast]

See also

const_cast conversion adds or removes const
static_cast conversion performs basic conversions
dynamic_cast conversion performs checked polymorphic conversions
reinterpret_cast conversion performs general low-level conversions
standard conversions implicit conversions from one type to another
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