Explicit type conversion

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function declaration
inline specifier
Exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
throw-expression	try-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Compound types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
Storage duration specifiers
Initialization
Default initialization Value initialization Zero initialization Copy initialization Direct initialization	Aggregate initialization List initialization (C++11) Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

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

Default comparisons(C++20)

Alternative representations of operators

Precedence and associativity

Fold expression(C++17)

new-expression

delete-expression

throw-expression

alignof

sizeof

sizeof...(C++11)

typeid

noexcept(C++11)

Operator overloading

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)

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,

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++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++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