Implicit conversions

Implicit conversions are performed whenever an expression of some type T1 is used in context that does not accept that type, but accepts some other type T2; in particular:

• when the expression is used as the argument when calling a function that is declared with T2 as parameter;
• when the expression is used as an operand with an operator that expects T2;
• when initializing a new object of type T2, including return statement in a function returning T2;
• when the expression is used in a switch statement (T2 is integral type);
• when the expression is used in an if statement or a loop (T2 is bool).

The program is well-formed (compiles) only if there exists one unambiguous implicit conversion sequence from T1 to T2.

If there are multiple overloads of the function or operator being called, after the implicit conversion sequence is built from T1 to each available T2, overload resolution rules decide which overload is compiled.

Note: in arithmetic expressions, the destination type for the implicit conversions on the operands to binary operators is determined by a separate set of rules: usual arithmetic conversions.

Order of the conversions

Implicit conversion sequence consists of the following, in this order:

When considering the argument to a constructor or to a user-defined conversion function, only one standard conversion sequence is allowed (otherwise user-defined conversions could be effectively chained). When converting from one non-class type to another non-class type, only a standard conversion sequence is allowed.

A standard conversion sequence consists of the following, in this order:

A user-defined conversion consists of zero or one non-explicit single-argument converting constructor or non-explicit conversion function call.

An expression e is said to be implicitly convertible to T2 if and only if T2 can be copy-initialized from e, that is the declaration T2 t = e; is well-formed (can be compiled), for some invented temporary t. Note that this is different from direct initialization (T2 t(e)), where explicit constructors and conversion functions would additionally be considered.

Contextual conversions

In the following contexts, a context-specific type T is expected, and the expression e of class type E is only allowed if.

Such expression e is said to be contextually implicitly converted to the specified type T. Note that explicit conversion functions are not considered, even though they are considered in contextual conversions to bool. (since C++11).

• the argument of the delete-expression (T is any object pointer type);
• integral constant expression, where a literal class is used (T is any integral or unscoped enumeration type, the selected user-defined conversion function must be constexpr);
• the controlling expression of the switch statement (T is any integral or enumeration type).

#include <cassert>
 
template<typename T>
class zero_init
{
    T val;
public:
    zero_init() : val(static_cast<T>(0)) {}
    zero_init(T val) : val(val) {}
    operator T&() { return val; }
    operator T() const { return val; }
};
 
int main()
{
    zero_init<int> i;
    assert(i == 0);
 
    i = 7;
    assert(i == 7);
 
    switch (i) {}     // error until C++14 (more than one conversion function)
                      // OK since C++14 (both functions convert to the same type int)
    switch (i + 0) {} // always okay (implicit conversion)
}

Value transformations

Value transformations are conversions that change the value category of an expression. They take place whenever an expression appears as an operand of an operator that expects an expression of a different value category.

Lvalue-to-rvalue conversion

An lvalue (until C++11)A glvalue (since C++11) of any non-function, non-array type T can be implicitly converted to an rvalue (until C++11)a prvalue (since C++11):

• If T is not a class type, the type of the rvalue (until C++11)prvalue (since C++11) is the cv-unqualified version of T.
• Otherwise, the type of the rvalue (until C++11)prvalue (since C++11) is T.

If an lvalue-to-rvalue conversion from an incomplete type is required by a program, that program is ill-formed.

This conversion models the act of reading a value from a memory location into a CPU register.

Array-to-pointer conversion

An lvalue or rvalue of type "array of N T" or "array of unknown bound of T" can be implicitly converted to a prvalue of type "pointer to T". If the array is a prvalue, temporary materialization occurs. (since C++17) The resulting pointer refers to the first element of the array (see array to pointer decay for details).

struct S { int m; };
int i = S().m; // member access expects glvalue as of C++17;
               // S() prvalue is converted to xvalue

Function-to-pointer conversion

An lvalue of function type T can be implicitly converted to a prvalue pointer to that function. This does not apply to non-static member functions because lvalues that refer to non-static member functions do not exist.

Numeric promotions

Integral promotion

prvalues of small integral types (such as char) may be converted to prvalues of larger integral types (such as int). In particular, arithmetic operators do not accept types smaller than int as arguments, and integral promotions are automatically applied after lvalue-to-rvalue conversion, if applicable. This conversion always preserves the value.

The following implicit conversions are classified as integral promotions:

• char, signed char, unsigned char, short and unsigned short can be converted to int if their respective entire value range can be held by the type int, or unsigned int otherwise;
• wchar_t, char8_t (since C++20), char16_t, and char32_t (since C++11) can be converted to the first type from the following list able to hold their entire value range:
• int
• unsigned int
• long
• unsigned long

• an unscoped enumeration type whose underlying type is not fixed can be converted to the first type from the following list able to hold their entire value range:
• int
• unsigned int
• long
• unsigned long

• an unscoped enumeration type whose underlying type is fixed can be converted to its underlying type, and, if the underlying type is also subject to integral promotion, to the promoted underlying type. Conversion to the unpromoted underlying type is better for the purposes of overload resolution;
• a bit-field type can be converted to int if it can represent entire value range of the bit-field, otherwise to unsigned int if it can represent entire value range of the bit-field, otherwise no integral promotions apply;
• the type bool can be converted to int with the value false becoming ​0​ and true becoming 1.

Note that all other conversions are not promotions; for example, overload resolution chooses char -> int (promotion) over char -> short (conversion).

Floating-point promotion

A prvalue of type float can be converted to a prvalue of type double. The value does not change.

Numeric conversions

Unlike the promotions, numeric conversions may change the values, with potential loss of precision.

Integral conversions

A prvalue of an integer type or of an unscoped enumeration type can be converted to any other integer type. If the conversion is listed under integral promotions, it is a promotion and not a conversion.

• If the destination type is unsigned, the resulting value is the smallest unsigned value equal to the source value modulo 2n
  where n is the number of bits used to represent the destination type.

• If the destination type is signed, the value does not change if the source integer can be represented in the destination type. Otherwise the result is implementation-defined (until C++20)the unique value of the destination type equal to the source value modulo 2n
  where n is the number of bits used to represent the destination type (since C++20) (note that this is different from signed integer arithmetic overflow, which is undefined).
• If the source type is bool, the value false is converted to zero and the value true is converted to the value one of the destination type (note that if the destination type is int, this is an integer promotion, not an integer conversion).
• If the destination type is bool, this is a boolean conversion (see below).

Floating-point conversions

If the conversion is listed under floating-point promotions, it is a promotion and not a conversion.

• If the source value can be represented exactly in the destination type, it does not change.
• If the source value is between two representable values of the destination type, the result is one of those two values (it is implementation-defined which one, although if IEEE arithmetic is supported, rounding defaults to nearest).
• Otherwise, the behavior is undefined.

Floating–integral conversions

• A prvalue of floating-point type can be converted to a prvalue of any integer type. The fractional part is truncated, that is, the fractional part is discarded. If the value cannot fit into the destination type, the behavior is undefined (even when the destination type is unsigned, modulo arithmetic does not apply). If the destination type is bool, this is a boolean conversion (see below).
• A prvalue of integer or unscoped enumeration type can be converted to a prvalue of any floating-point type. The result is exact if possible. If the value can fit into the destination type but cannot be represented exactly, it is implementation defined whether the closest higher or the closest lower representable value will be selected, although if IEEE arithmetic is supported, rounding defaults to nearest. If the value cannot fit into the destination type, the behavior is undefined. If the source type is bool, the value false is converted to zero, and the value true is converted to one.

Pointer conversions

• A null pointer constant (see NULL), can be converted to any pointer type, and the result is the null pointer value of that type. Such conversion (known as null pointer conversion) is allowed to convert to a cv-qualified type as a single conversion, that is, it's not considered a combination of numeric and qualifying conversions.
• A prvalue pointer to any (optionally cv-qualified) object type T can be converted to a prvalue pointer to (identically cv-qualified) void. The resulting pointer represents the same location in memory as the original pointer value. If the original pointer is a null pointer value, the result is a null pointer value of the destination type.
• A prvalue pointer to a (optionally cv-qualified) derived complete class type can be converted to a prvalue pointer to its (identically cv-qualified) base class. If the base class is inaccessible or ambiguous, the conversion is ill-formed (won't compile). The result of the conversion is a pointer to the base class subobject within the pointed-to object. The null pointer value is converted to the null pointer value of the destination type.

Pointer-to-member conversions

• A null pointer constant (see NULL) can be converted to any pointer-to-member type, and the result is the null member pointer value of that type. Such conversion (known as null member pointer conversion) is allowed to convert to a cv-qualified type as a single conversion, that is, it's not considered a combination of numeric and qualifying conversions.
• A prvalue pointer to member of some type T in a base class B can be converted to a prvalue pointer to member of the same type T in its derived complete class D. If B is inaccessible, ambiguous, or virtual base of D or is a base of some intermediate virtual base of D, the conversion is ill-formed (won't compile). The resulting pointer can be dereferenced with a D object, and it will access the member within the B base subobject of that D object. The null pointer value is converted to the null pointer value of the destination type.

Boolean conversions

A prvalue of integral, floating-point, unscoped enumeration, pointer, and pointer-to-member types can be converted to a prvalue of type bool.

The value zero (for integral, floating-point, and unscoped enumeration) and the null pointer and the null pointer-to-member values become false. All other values become true.

Qualification conversions

• A prvalue of type pointer to cv-qualified type T can be converted to a prvalue pointer to a more cv-qualified same type T (in other words, constness and volatility can be added).
• A prvalue of type pointer to member of cv-qualified type T in class X can be converted to a prvalue pointer to member of more cv-qualified type T in class X.

"More" cv-qualified means that.

• a pointer to unqualified type can be converted to a pointer to const;
• a pointer to unqualified type can be converted to a pointer to volatile;
• a pointer to unqualified type can be converted to a pointer to const volatile;
• a pointer to const type can be converted to a pointer to const volatile;
• a pointer to volatile type can be converted to a pointer to const volatile.

For multi-level pointers, the following restrictions apply: a multilevel pointer P1 which is cv1
0-qualified pointer to cv1
1-qualified pointer to ... cv1
n-1-qualified pointer to cv1
n-qualified T is convertible to a multilevel pointer P2 which is cv2
0-qualified pointer to cv2
1-qualified pointer to ... cv2
n-1-qualified pointer to cv2
n-qualified T only if.

• the number of levels n is the same for both pointers;

• if there is a const in the cv1
  k qualification at some level (other than level zero) of P1, there is a const in the same level cv2
  k of P2;
• if there is a volatile in the cv1
  k qualification at some level (other than level zero) of P1, there is a volatile in the same cv2
  k level of P2;

• if at some level k the P2 is more cv-qualified than P1 or there is an array type of known bound in P1 and an array type of unknown bound in P2 (since C++20), then there must be a const at every single level (other than level zero) of P2 up until k: cv2
  1, cv2
  2 ... cv2
  k.
• same rules apply to multi-level pointers to members and multi-level mixed pointers to objects and pointers to members;
• same rules apply to multi-level pointers that include pointers to array of known or unknown bound at any level (arrays of cv-qualified elements are considered to be identically cv-qualified themselves);
• level zero is addressed by the rules for non-multilevel qualification conversions.

char** p = 0;
const char** p1 = p; // error: level 2 more cv-qualified but level 1 is not const
const char* const * p2 = p; // OK: level 2 more cv-qualified and const added at level 1
volatile char * const * p3 = p; // OK: level 2 more cv-qual and const added at level 1
volatile const char* const* p4 = p2; // OK: 2 more cv-qual and const was already at 1
 
double *a[2][3];
double const * const (*ap)[3] = a; // OK
double * const (*ap1)[] = a;       // OK since C++20

Note that in the C programming language, const/volatile can be added to the first level only:

char** p = 0;
char * const* p1 = p;       // OK in C and C++
const char* const * p2 = p; // error in C, OK in C++

void (*p)();
void (**pp)() noexcept = &p; // error: cannot convert to pointer to noexcept function
 
struct S
{
    typedef void (*p)();
    operator p();
};
void (*q)() noexcept = S(); // error: cannot convert to pointer to noexcept function

The safe bool problem

Until the introduction of explicit conversion functions in C++11, designing a class that should be usable in boolean contexts (e.g. if (obj) { ... }) presented a problem: given a user-defined conversion function, such as T::operator bool() const;, the implicit conversion sequence allowed one additional standard conversion sequence after that function call, which means the resultant bool could be converted to int, allowing such code as obj << 1; or int i = obj;.

One early solution for this can be seen in std::basic_ios, which initially defines operator void*, so that the code such as if (std::cin) {...} compiles because void* is convertible to bool, but int n = std::cout; does not compile because void* is not convertible to int. This still allows nonsense code such as delete std::cout; to compile.

Many pre-C++11 third party libraries were designed with a more elaborate solution, known as the Safe Bool idiom. std::basic_ios also allowed this idiom via LWG issue 468, and operator void* was replaced (see notes).

explicit operator bool() const { ... }

Footnotes

1. This only applies if the arithmetic is two's complement which is only required for the exact-width integer types. Note, however, that at the moment all platforms with a C++ compiler use two's complement arithmetic

Defect reports

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

See also

• const_cast
• static_cast
• dynamic_cast
• reinterpret_cast
• explicit cast
• user-defined conversion