When an expression is used in the context where a value of a different type is expected, conversion may occur:
int n = 1L; // expression 1L has type long, int is expected n = 2.1; // expression 2.1 has type double, int is expected char *p = malloc(10); // expression malloc(10) has type void*, char* is expected
Conversions take place in the following situations:
return is converted to an object having the return type of the function Note that actual assignment, in addition to the conversion, also removes extra range and precision from floating-point types and prohibits overlaps; those characteristics do not apply to conversion as if by assignment.
In a function call expression when the call is made to.
Each argument of integer type undergoes integer promotion (see below), and each argument of type float is implicitly converted to the type double.
int add_nums(int count, ...); int sum = add_nums(2, 'c', true); // add_nums is called with three ints: (2, 99, 1)
| Note that | (since C99) |
The arguments of the following arithmetic operators undergo implicit conversions for the purpose of obtaining the common real type, which is the type in which the calculation is performed:
long double, long double complex, or long double imaginary (since C99), the other operand is implicitly converted as follows: long double | (since C99) |
double, double complex, or double imaginary (since C99), the other operand is implicitly converted as follows: double | (since C99) |
float, float complex, or float imaginary (since C99), the other operand is implicitly converted as follows: float (the only real type possible is float, which remains as-is) | (since C99) |
1.f + 20000001; // int is converted to float, giving 20000000.00
// addition and then rounding to float gives 20000000.00
(char)'a' + 1L; // first, char 'a', which is 97, is promoted to int
// different types: int and long
// same signedness: both signed
// different rank: long is of greater rank than int
// therefore, int 97 is converted to long 97
// the result is 97 + 1 = 98 of type signed long
2u - 10; // different types: unsigned int and signed int
// different signedness
// same rank
// therefore, signed int 10 is converted to unsigned int 10
// since the arithmetic operation is performed for unsigned integers (see "Arithmetic operators" topic),
// the calculation performed is (2 - 10) modulo (2 raised to n), where n is the number of bits of unsigned int
// if unsigned int is 32-bit long, then
// the result is (-8) modulo (2 raised to 32) = 4294967288 of type unsigned int
5UL - 2ULL; // different types: unsigned long and unsigned long long
// same signedness
// different rank: rank of unsigned long long is greater
// therefore, unsigned long 5 is converted to unsigned long long 5
// since the arithmetic operation is performed for unsigned integers (see "Arithmetic operators" topic),
// if unsigned long long is 64-bit long, then
// the result is (5 - 2) modulo (2 raised to 64) = 3 of type unsigned long long
0UL - 1LL; // different types: unsigned long and signed long long
// different signedness
// different rank: rank of signed long long is greater.
// if ULONG_MAX > LLONG_MAX, then signed long long cannot represent all unsigned long
// therefore, this is the last case: both operands are converted to unsigned long long
// unsigned long 0 is converted to unsigned long long 0
// long long 1 is converted to unsigned long long 1
// since the arithmetic operation is performed for unsigned integers (see "Arithmetic operators" topic),
// if unsigned long long is 64-bit long, then
// the calculation is (0 - 1) modulo (2 raised to 64)
// thus, the result is 18446744073709551615 (ULLONG_MAX) of type unsigned long long| The result type is determined as follows:
double complex z = 1 + 2*I; double f = 3.0; z + f; // z remains as-is, f is converted to double, the result is double complex | (since C99) |
As always, the result of a floating-point operator may have greater range and precision than is indicated by its type (see FLT_EVAL_METHOD).
| Note: real and imaginary operands are not implicitly converted to complex because doing so would require extra computation, while producing undesirable results in certain cases involving infinities, NaNs and signed zeros. For example, if reals were converted to complex, 2.0×(3.0+i∞) would evaluate as (2.0+i0.0)×(3.0+i∞) ⇒ (2.0×3.0–0.0×∞) + i(2.0×∞+0.0×3.0) ⇒ NaN+i∞ rather than the correct 6.0+i∞. If imaginaries were converted to complex, i2.0×(∞+i3.0) would evaluate as (0.0+i2.0) × (∞+i3.0) ⇒ (0.0×∞ – 2.0×3.0) + i(0.0×3.0 + 2.0×∞) ⇒ NaN + i∞ instead of –6.0 + i∞. | (since C99) |
Note: regardless of usual arithmetic conversions, the calculation may always be performed in a narrower type than specified by these rules under the as-if rule.
Any lvalue expression of any non-array type, when used in any context other than.
undergoes lvalue conversion: the type remains the same, but loses const/volatile/restrict-qualifiers and atomic properties, if any. The value remains the same, but loses its lvalue properties (the address may no longer be taken).
If the lvalue has incomplete type, the behavior is undefined.
If the lvalue designates an object of automatic storage duration whose address was never taken and if that object was uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.
This conversion models the memory load of the value of the object from its location.
volatile int n = 1; int x = n; // lvalue conversion on n reads the value of n volatile int* p = &n; // no lvalue conversion: does not read the value of n
Any lvalue expression of array type, when used in any context other than.
undergoes a conversion to the non-lvalue pointer to its first element.
If the array was declared register, the behavior is undefined.
int a[3], b[3][4]; int* p = a; // conversion to &a[0] int (*q)[4] = b; // conversion to &b[0]
Any function designator expression, when used in any context other than.
undergoes a conversion to the non-lvalue pointer to the function designated by the expression.
int f(int); int (*p)(int) = f; // conversion to &f (***p)(1); // repeated dereference to f and conversion back to &f
Implicit conversion, whether as if by assignment or a usual arithmetic conversion, consists of two stages:
Conversion of a value of any type to any compatible type is always a no-op and does not change the representation.
uint8_t (*a)[10]; // if uint8_t is a typedef to unsigned char unsigned char (*b)[] = a; // then these pointer types are compatible
Integer promotion is the implicit conversion of a value of any integer type with rank less or equal to rank of int or of a bit field of type _Bool, int, signed int, unsigned int, to the value of type int or unsigned int.
If int can represent the entire range of values of the original type (or the range of values of the original bit field), the value is converted to type int. Otherwise the value is converted to unsigned int.
Integer promotions preserve the value, including the sign:
int main(void) {
void f(); // old-style function declaration
char x = 'a'; // integer conversion from int to char
f(x); // integer promotion from char back to int
}
void f(x) int x; {} // the function expects intrank above is a property of every integer type and is defined as follows:
signed char < rank of short < rank of int < rank of long int < rank of long long int
__int64 < rank of long long int, but rank of long long < rank of __int128 due to the rule (1))signed char and rank of unsigned char
_Bool is less than the rank of any other standard integer typeNote: integer promotions are applied only.
+ and - ~ << and >> Boolean conversionA value of any scalar type (including bool b1 = 0.5; // b1 == 1 (0.5 converted to int would be zero) bool b2 = 2.0*_Imaginary_I; // b2 == 1 (but converted to int would be zero) bool b3 = 0.0 + 3.0*I; // b3 == 1 (but converted to int would be zero) bool b4 = 0.0/0.0; // b4 == 1 (NaN does not compare equal to zero) bool b5 = nullptr; // b5 == 0 (since C23: nullptr is converted to false) | (since C99) |
A value of any integer type can be implicitly converted to any other integer type. Except where covered by promotions and boolean conversions above, the rules are:
char x = 'a'; // int -> char, result unchanged
unsigned char n = -123456; // target is unsigned, result is 192 (that is, -123456+483*256)
signed char m = 123456; // target is signed, result is implementation-defined
assert(sizeof(int) > -1); // assert fails:
// operator > requests conversion of -1 to size_t,
// target is unsigned, result is SIZE_MAXA finite value of any real floating type can be implicitly converted to any integer type. Except where covered by boolean conversion above, the rules are:
int n = 3.14; // n == 3 int x = 1e10; // undefined behavior for 32-bit int
A value of any integer type can be implicitly converted to any real floating type.
FE_INEXACT is raised in this case. FE_INVALID is raised and the result value is unspecified. The result of this conversion may have greater range and precision than its target type indicates (see FLT_EVAL_METHOD.
If control over FE_INEXACT is needed in floating-to-integer conversions, rint and nearbyint may be used.
double d = 10; // d = 10.00 float f = 20000001; // f = 20000000.00 (FE_INEXACT) float x = 1+(long long)FLT_MAX; // undefined behavior
A value of any real floating type can be implicitly converted to any other real floating type.
The result of this conversion may have greater range and precision than its target type indicates (see FLT_EVAL_METHOD.
double d = 0.1; // d = 0.1000000000000000055511151231257827021181583404541015625 float f = d; // f = 0.100000001490116119384765625 float x = 2*(double)FLT_MAX; // undefined
Complex type conversionsA value of any complex type can be implicitly converted to any other complex type. The real part and the imaginary part individually follow the conversion rules for the real floating types. double complex d = 0.1 + 0.1*I; float complex f = d; // f is (0.100000001490116119384765625, 0.100000001490116119384765625) Imaginary type conversionsA value of any imaginary type can be implicitly converted to any other imaginary type. The imaginary part follows the conversion rules for the real floating types. double imaginary d = 0.1*_Imaginary_I; float imaginary f = d; // f is 0.100000001490116119384765625*I Real-complex conversionsA value of any real floating type can be implicitly converted to any complex type.
A value of any complex type can be implicitly converted to any real floating type.
Note: in complex-to-real conversion, a NaN in the imaginary part will not propagate to the real result. double complex z = 0.5 + 3*I; float f = z; // the imaginary part is discarded, f is set to 0.5 z = f; // sets z to 0.5 + 0*I Real-imaginary conversionsA value of any imaginary type can be implicitly converted to any real type (integer or floating-point). The result is always a positive (or unsigned) zero, except when the target type is _Bool, in which case boolean conversion rules apply. A value of any real type can be implicitly converted to any imaginary type. The result is always a positive imaginary zero. double imaginary z = 3*I; bool b = z; // Boolean conversion: sets b to true float f = z; // Real-imaginary conversion: sets f to 0.0 z = 3.14; // Imaginary-real conversion: sets z to 0*_Imaginary_I Complex-imaginary conversionsA value of any imaginary type can be implicitly converted to any complex type.
A value of any complex type can be implicitly converted to any imaginary type.
double imaginary z = I * (3*I); // the complex result -3.0+0i loses real part
// sets z to 0*_Imaginary_I | (since C99) |
A pointer to void can be implicitly converted to and from any pointer to object type with the following semantics:
int* p = malloc(10 * sizeof(int)); // malloc returns void*
A pointer to an unqualified type may be implicitly converted to the pointer to qualified version of that type (in other words, const, volatile, and restrict qualifiers can be added. The original pointer and the result compare equal.
int n; const int* p = &n; // &n has type int*
Any integer constant expression with value 0 as well as integer pointer expression with value zero cast to the type void* can be implicitly converted to any pointer type (both pointer to object and pointer to function). The result is the null pointer value of its type, guaranteed to compare unequal to any non-null pointer value of that type. This integer or void* expression is known as null pointer constant and the standard library provides one definition of this constant as the macro NULL .
int* p = 0; double* q = NULL;
Although signed integer overflow in any arithmetic operator is undefined behavior, overflowing a signed integer type in an integer conversion is merely unspecified behavior.
On the other hand, although unsigned integer overflow in any arithmetic operator (and in integer conversion) is a well-defined operation and follows the rules of modulo arithmetic, overflowing an unsigned integer in a floating-to-integer conversion is undefined behavior: the values of real floating type that can be converted to unsigned integer are the values from the open interval (-1; Unnn_MAX+1).
unsigned int n = -1.0; // undefined behavior
Conversions between pointers and integers (except from pointer to _Bool and (since C99)from integer constant expression with the value zero to pointer), between pointers to objects (except where either to or from is a pointer to void) and conversions between pointers to functions (except when the functions have compatible types) are never implicit and require a cast operator.
There are no conversions (implicit or explicit) between pointers to functions and pointers to objects (including void*) or integers.
| C++ documentation for Implicit conversions |
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