More C++ Idioms/Implicit conversions

Implicit conversions

Intent

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.

Motivation

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 en.cppreference.com implicit_conversion

Solution and Sample Code

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

Everywhere, all the time, ...