CODEWITHSUNDEEP
cgccc11
Scipio
Scipio

Reputation: 171

How to use const int in _Static_assert?

Preamble: I want to statically check amount of struct members in C program, so I created two macros, each of them creates constant int storing __LINE__ into variable:

 #include <stdio.h>
 #include <string.h>
 #define BEGIN(log) const int __##log##_begin = __LINE__;
 #define END(log) const int __##log##_end = __LINE__;
 BEGIN(TEST);
 struct TEST {
    int t1;
    int t2;
    float t3;
    int t4;
    int t5;
    int t6;
 };
 END(TEST)

 main()
 {
    static_assert(__TEST_end - __TEST_begin  == 6 + 3, "not_equal");
 }

When I use C++ compiler with -std=c++11 option (c++ test.cpp -std=c++11), it works fine, but the same code (with replacement of static_assert to _Static_assert) doesn't work in C(gcc version 4.8.4) with a strange error as this expression could be evaluated at a compile time:

test.c: In function ‘main’: test.c:18:17: error: expression in static assertion is not constant _Static_assert(__TEST_end - __TEST_begin == 6 + 4, "not_equal");

How can I fix this error or achieve the original goal in C?

Upvotes: 5

Views: 3284

Answers (2)

Simon Kissane
Simon Kissane

Reputation: 5278

A solution for your issue would be to use an anonymous enumeration. Instead of:

#define BEGIN(log) const int __##log##_begin = __LINE__
#define END(log) const int __##log##_end = __LINE__

Do:

#define BEGIN(log) enum { __##log##_begin = __LINE__ }
#define END(log) enum { __##log##_end = __LINE__ }

This is allowed in C11 since, unlike a const int (or even static const int) variable, an enumeration constant is defined to be an integral constant expression.

(As an aside, I omitted the terminal semicolon from my version of your BEGIN()/END() macros. In my opinion, declaration macros should not include a terminal semicolon, that should be provided by the macro user, so the macro behaves more like a non-macro C declaration.)

Upvotes: 0

this
this

Reputation: 5290

In C a variable even if defined with const is not a constant expression. _Static_assert requires its first parameter to be a constant expression. Therefore the same thing that can be done in C++ cannot be done in C.

You can do a runtime check instead; use assert.

Note that this method won't guard you against a programmer typing out two members in the same line or using multiple single line declarations of the same type, or adding an empty line (or a comment). Instead of forcing a programmer to follow a string coding pattern, just so that this assert will catch the error, it is less error prone to simply require the programmer to define a correct number of members. It is strictly better, because you can make the an undetectable error either way, but at least doesn't have to worry about the strict coding pattern.

Upvotes: 6

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