Implicit conversions

Implicit conversions are automatically performed when a value is copied to a compatible type. For example:
short a=2000;
int b;
Here, the value of a is promoted from short to int without the need of any explicit operator. This is known as a standard conversion. Standard conversions affect fundamental data types, and allow the conversions between numerical types (short to int, int to float, double to int...), to or from bool, and some pointer conversions.
Converting to int from some smaller integer type, or to double from float is known as promotion, and is guaranteed to produce the exact same value in the destination type. Read more



No-promotion arithmetic types conversions

No-promotion conversions between arithmetic types may not always be able to represent the same value exactly:
  • negative integer to an unsigned type, the resulting value corresponds to its 2's complement bitwise representation (i.e., -1 becomes the largest value representable by the type, -2 the second largest, ...).
  • conv. from/to bool consider false equivalent to zero (for numeric types) and to null pointer (for pointer types); true is equivalent to all other values and is converted to the equivalent of 1.
  • floating-point type to an integer type, the value is truncated (the decimal part is removed). If the result lies outside the range of representable values by the type, the conversion causes undefined behavior.
  • if the conv. is between numeric types of the same kind, the conversion is valid, but the value is implementation-specific (may not be portable).
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Types conversions

No-promotion conversions may imply a loss of precision, which the compiler can signal with a warning. This warning can be avoided with an explicit conversion.
For non-fundamental types, arrays and functions implicitly convert to pointers, and pointers in general allow the following conversions:
  • Null pointers can be converted to pointers of any type
  • Pointers to any type can be converted to void pointers.
  • Pointer upcast: pointers to a derived class can be converted to a pointer of an accessible and unambiguous base class, without modifying its const or volatile qualification.
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data-type-conversion promotion

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