CODEWITHSUNDEEP
c++language-lawyer
user8221510
user8221510

Reputation:

Why is there a so-called identity conversion?

If an identity conversion does not do anything (whenever I have encountered it in the standard, it was simply to tell that there was no need for conversion), why is it a thing? What is its true purpose?

Exmaples of its usage

From the standard (n3690 13.3.3.1.1):

As described in Clause 4, a standard conversion sequence is either the Identity conversion by itself (that is, no conversion) or consists of one to three conversions from the other four categories.

13.3.3.1:

If no conversions are required to match an argument to a parameter type, the implicit conversion sequence is the standard conversion sequence consisting of the identity conversion

cppreference:

Otherwise, if the parameter type is an aggregate which can be initialized from the initializer list according by aggregate initialization, the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion.

struct A { int m1; double m2;};
void f(A);
f({'a','b'});  // calls f(A(int,double)), user-defined conversion

"with the second standard conversion sequence an identity conversion" - is that even worth mentioning?

Upvotes: 0

Views: 524

Answers (1)

Johan
Johan

Reputation: 3871

The purpose of the identity conversion is to act as a base case and allow us to reason about conversions without having to single out the case where no conversion is needed.

Upvotes: 2

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