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c++assertkeyword
Ricky65
Ricky65

Reputation: 1687

In C++ why isn't "assert" a keyword?

Now "static_assert" is a keyword in C++0x I thought it would be logical to replace the C "assert" macro with an "assert" keyword too.

Upvotes: 7

Views: 847

Answers (7)

Qualia
Qualia

Reputation: 729

The other answers give some possible answers to your question, but a recent proposal indicates that assert may indeed become a keyword in C++17: https://isocpp.org/files/papers/N4154.pdf

Upvotes: 2

Puppy
Puppy

Reputation: 146940

Basically, because it doesn't need it. Existing assertion mechanisms for run-time assertions are perfectly good and don't require language support.

Upvotes: 2

sergio
sergio

Reputation: 69027

In C++0x (from here):

In C++0x, static assertions can be declared to detect and diagnose common usage errors at compile time.

this is static_assert syntax:

>>-static_assert--(--constant-expression--,--string-literal----->

where constant-expression must be contextually converted to bool. If it converts to false, then the compiler will emit an error according the string-literal.

So, this is basically an extension of the language that needs a keyword. It is not a runtime mechanism.

Again from the document linked above:

The addition of static assertions to the C++ language has the following benefits:

  1. Libraries can detect common usage errors at compile time.

  2. Implementations of the C++ Standard Library can detect and diagnose common usage errors, improving usability.

  3. You can use a static_assert declaration to check important program invariants at compile time.

Upvotes: 1

Dennis Zickefoose
Dennis Zickefoose

Reputation: 10969

assert has no compile time meaning, except during Pre-processing. The preprocessor has no knowledge of the C++ language, so a keyword makes no sense.

By contrast, static_assert is evaluated at compile time. Making it a keyword makes more sense in that regard. The compiler cares about it's existence.

There are also historic reasons; it was not a keyword in C, and making it one in C++ would have rendered existing assert macros result in undefined behavior.

Upvotes: 2

Nik
Nik

Reputation: 695

This can not be done for compatibility with the code already written in c which has assert as a variable name. And hence as oli mentioned we won't be able to compile as assert is no longer macro

Upvotes: 1

Lawrence McAlpin
Lawrence McAlpin

Reputation: 2785

static_assert is interpreted at compile time so it has to be a keyword so the compiler can process it.

assert doesn't need to be a keyword, and it doesn't make much sense to make it one, since there are many ways a program might want to respond to assertion success or failure. Therefore, it makes more sense to implement it in a library, and it is typically implemented as a macro.

Upvotes: 3

ildjarn
ildjarn

Reputation: 62975

assert can be implemented in a library, static_assert cannot. So static_assert gets a keyword because it needs language support, and assert doesn't.

Upvotes: 1

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