More C++ Idioms/Implicit conversions

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Implicit conversions[edit]

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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.

Also Known As[edit]


In some contexts a variable can be used which is not exactly the type required by the function. 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.

More info on: C++ Reference implicit_conversion

Solution and Sample Code[edit]

Conversion of pointer to boolean:

int a = 42;
int* ptr = &a;
if (ptr) // checks if ptr is not null_ptr

Conversion of std::string to some other type:

#include <string>

struct A {
    A( const std::string & s ) {}

void func( const A & a ) {

int main() {
    func( "one" );               // error - requires 2 steps to convert: const char* -> std::string -> A
    func( A("two") );            // ok - converting const char* -> std::string, which is used to create A
    func( std::string("three") );// ok - implicit conversion std::string -> A

Example comes from this Stack overflow question titled: C++ implicit conversions.

Known Uses[edit]

Everywhere, all the time, ...

Related Idioms[edit]