CODEWITHSUNDEEP
c++syntax
ZijingWu
ZijingWu

Reputation: 3490

Why static_assert is a declaration instead of a postfix-expression in C++ syntax?

I just perceived that static_assert is defined as a static_assert-declaration.

By my understanding, the declaration should introduce a name, function name/object name or type name. static_assert doesn't fit into this. This is not intuitive.

So why is static_assert defined as a declaration instead of a postfix-expression statement?

Follows the relevant part of the standard:

static_assert-declaration:
     static_assert ( constant-expression , string-literal ) ;

Upvotes: 2

Views: 191

Answers (1)

AnT stands with Russia
AnT stands with Russia

Reputation: 320719

Firstly, the big difference between static assertions and classic regular assert is that assert is an executable assertion. It is a run-time assertion that has to be executed in order to do its job. For this reason, it has to be a statement or an expression. (It happens to be an expression.) Meanwhile, static assertion works completely differently: is not an executable assertion, it is a compile-time assertion. So, it is not naturally restricted to being a statement or an expression.

Secondly, it has to be able to appear where expressions are not allowed to appear by themselves - in namespace scope and among class member declarations. In order to support such placement it has to be either a declaration or something completely new. The language authors decided not to introduce a new kind of entity and simply made it a declaration.

Thirdly, in C++11 a declaration does not necessarily introduce a name. For example, C++11 supports empty declaration and attribute declaration, neither of which introduces names. Specifically for that reason in C++11 the description of the basic concept of declaration was changed from

A declaration introduces names into a translation unit or redeclares names introduced by previous declarations. (C++03)

to

A declaration may introduce one or more names into a translation unit or redeclare names introduced by previous declarations. (C++11)

Upvotes: 6

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