Explicit type conversion

 

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

Syntax

( type-id ) unary-expression	(1)
simple-type-specifier ( expression-list (optional) ) simple-type-specifier ( initializer-list (optional) )	(2)	(until C++11) (since C++11)
simple-type-specifier { initializer-list (optional) }	(3)	(since C++11)
simple-type-specifier { designated-initializer-list }	(4)	(since C++20)
typename identifier ( initializer-list (optional) )	(5)	(since C++11)
typename identifier { initializer-list (optional) }	(6)	(since C++11)
typename identifier { designated-initializer-list }	(7)	(since C++20)

Explicitly converts any number of values to a value of the target type.

1) Explicit type conversion (cast notation), also called C-style cast.

2-7) Explicit type conversion (functional notation), also called function-style cast.

type-id	-	a type-id
unary-expression	-	an unary expression (whose top-level operator does not have a precedence higher than that of C-style cast)
simple-type-specifier	-	a simple type specifier
expression-list	-	a comma-separated list of expressions (except unparenthesized comma expressions)
initializer-list	-	a comma-separated list of initializer clauses
designated-initializer-list	-	a comma-separated list of designated initializer clauses
identifier	-	a (possibly qualified) identifier (including template identifiers)

Explanation

1) When the C-style cast is encountered, the compiler attempts to interpret it as the following cast expressions, in this order:

a) const_cast<type-id >(unary-expression );

b) static_cast<type-id >(unary-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) a static_cast (with extensions) followed by const_cast;

d) reinterpret_cast<type-id >(unary-expression );

e) a reinterpret_cast followed by const_cast.

The first choice that satisfies the requirements of the respective cast operator is selected, even if it is ill-formed (see example). If a static_cast followed by a const_cast is used and the conversion can be interpreted in more than one way as such, the conversion is ill-formed.

In addition, C-style casts can cast from, to, and between pointers to incomplete class type. If both type-id and the type of unary-expression are pointers to incomplete class types, it is unspecified whether static_cast or reinterpret_cast gets selected.

2-7) A function-style cast specifies a type (simple-type-specifier  or identifier (since C++11)) and an initializer (the remaining parts), it constructs a value of the target type T, which is determined from the specified type and initializer(since C++17):

T is the specified type.	(until C++17)
T is determined as follows: If the specified type is a placeholder for a deduced class type, T is the return type of the function selected by overload resolution for class template deduction. Otherwise, if the specified type contains a placeholder type, T is the deduced type. (since C++23) Otherwise, T is the specified type.	(since C++17)

The conversion result is determined as follows:

• If the function-style cast is of syntax (2), and there is exactly one expression in parentheses, this cast is equivalent to the corresponding C-style cast.
• Otherwise, if T is (possibly cv-qualified) void, the result is an rvalue(until C++11)a prvalue(since C++11) of type void that performs no initialization.

If the initializer is not (), the program is ill-formed.	(until C++11)
If the initializer is not () or {} after pack expansion (if any), the program is ill-formed.	(since C++11)

• Otherwise, if T is a reference type, the function-style cast has the same effect as direct-initializing an invented variable t of type T from the specified initializer, and the result is the initialized t.

The result is an lvalue.	(until C++11)
If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue. Otherwise, the result is an xvalue.	(since C++11)

• Otherwise, the result is an rvalue(until C++11)a prvalue(since C++11) of of type T designating a temporary(until C++17)whose result object is(since C++17) direct-initialized with the specified initializer.

Ambiguity Resolution

Ambiguous declaration statement

In the case of an ambiguity between an 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, equivalent to L n;
    L(l)(m); // still a declaration, equivalent to L l((m));
}

However, if the outermost declarator in the ambiguous declaration statement has a trailing return type, the statement will only be treated as a declaration statement if the trailing return type starts with auto: struct M;   struct S { S* operator()(); int N; int M;   void mem(S s) { auto(s)()->M; // expression (S::M hides ::M), invalid before C++23 } };   void f(S s) { { auto(s)()->N; // expression, invalid before C++23 auto(s)()->M; // function declaration, equivalent to M s(); } { S(s)()->N; // expression S(s)()->M; // expression } }

(since C++11)

Ambiguous function parameter

The ambiguity above can also occur in the context of a declaration. In that context, the choice is between an object declaration with a function-style cast as the initializer and a declaration involving a function declarator with a redundant set of parentheses around a parameter name. The resolution is also to consider any construct, such as the potential parameter declaration, that could possibly be a declaration to be 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(*)() 
                 // that is adjusted from 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
}

However, if the outermost declarator in the ambiguous parameter declaration has a trailing return type, the ambiguity will only be resolved by treating it as a declaration if it starts with auto: typedef struct BB { int C[2]; } *B, C;   void foo() { S a(B()->C); // object declaration: B()->C cannot declare a parameter S b(auto()->C); // function declaration: has an unnamed parameter of type C(*)() // that is adjusted from C() }

(since C++11)

Ambiguous type-id

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)
}

However, if the outermost abstract-declarator in the ambiguous type-id has a trailing return type, the ambiguity will only be resolved by treating it as a type-id if it starts with auto: typedef struct BB { int C[2]; } *B, C;   void foo() { sizeof(B()->C[1]); // OK, sizeof(expression) sizeof(auto()->C[1]); // error: sizeof of a function returning an array }

(since C++11)

Notes

Feature-test macro	Value	Std	Feature
__cpp_auto_cast	202110L	(C++23)	auto(x) and auto{x}

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

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 1223 (P2915R0)	C++11	the addition of trailing return type introduced more ambiguities	resolves them
CWG 1893	C++11	function-style cast did not consider pack expansions	considers them
CWG 2351	C++11	void{} was ill-formed	made well-formed
CWG 2620	C++98	the resolution of ambiguous function parameters might be misinterpreted	improved the wording
CWG 2828	C++98	a C-style cast was ill-formed if multiple interpretations of a static_cast followed by a const_cast exist, regardless of whether these conversions are actually used	only considers the conversions possibly being used
CWG 2894	C++98	function-style casts could create reference rvalues	can only create reference lvalues

References

• C++23 standard (ISO/IEC 14882:2024):

• 7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]

• 7.6.3 Explicit type conversion (cast notation) [expr.cast]

• 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
C documentation for cast operator