Explicit type conversion

 

C++

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Technical specifications

Symbols index

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C++ language

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Expressions

General

value categories (lvalue, rvalue, xvalue)

order of evaluation (sequence points)

constant expressions

unevaluated expressions

primary expressions

lambda-expression(C++11)

Literals

integer literals

floating-point literals

boolean literals

character literals including escape sequences

string literals

null pointer literal(C++11)

user-defined literal(C++11)

Operators

Assignment operators: a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b

Increment and decrement: ++a, --a, a++, a--

Arithmetic operators: +a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b

Logical operators: a||b, a&&b, !a

Comparison operators: a==b, a!=b, a<b, a>b, a<=b, a>=b, a<=>b(C++20)

Member access operators: a[b], *a, &a, a->b, a.b, a->*b, a.*b

Other operators: a(...), a,b, a?b:c

new-expression

delete-expression

throw-expression

alignof

sizeof

sizeof...(C++11)

typeid

noexcept(C++11)

Fold expression(C++17)

Alternative representations of operators

Precedence and associativity

Operator overloading

Default comparisons(C++20)

Conversions

Implicit conversions

const_cast

static_cast

reinterpret_cast

dynamic_cast

Explicit conversions (T)a, T(a)

User-defined conversion

 

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

Syntax

( new-type ) expression	(1)
new-type ( expression )	(2)
new-type ( arg1, arg2, ... )	(3)
new-type ( )	(4)
new-type { arg1, arg2, ...(optional) }	(5)	(since C++11)
template-name ( arg1, arg2, ...(optional) )	(6)	(since C++17)
template-name { arg1, arg2, ...(optional) }	(7)	(since C++17)
auto ( expression )	(8)	(since C++23)
auto { expression }	(9)	(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 cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name: unsigned int(expression) or int*(expression) are not valid), followed by a single expression in parentheses. This cast expression is exactly equivalent to the corresponding C-style cast expression.

3) 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.

4) 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).

5) 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)

6,7) Same as (2-5), except first performs class template argument deduction.

8,9) 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 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.

Example

Run this code

double f = 3.14;
unsigned int n1 = (unsigned int)f; // C-style cast
unsigned int n2 = unsigned(f);     // functional cast
 
class C1;
class C2;
C2* foo(C1* p)
{
    return (C2*)p; // casts incomplete type to incomplete type
}
 
// 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
}

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
C documentation for cast operator