2

There is such code:

#include <assert.h>

void fun1()
{
   int x;
}

int main()
{
   assert(true
         ==true);
   return 0;
}

void fun2()
{
   int y;
}

There are two unused variables x and y which should be reported by a compiler when option -Wall is passed.

Let's first run only preprocessor:

g++ main.cpp -Wall -E -o main2.cpp

The output:

a lot of stuff from assert.h
...
# 2 "main.cpp" 2

void fun1()
{
   int x;
}

int main()
{
   ((true ==true) ? static_cast<void> (0) : __assert_fail ("true ==true",
 "main.cpp"
# 10 "main.cpp" 3 4
   ,
 11
# 10 "main.cpp" 3 4
   , __PRETTY_FUNCTION__))
                ;
   return 0;
}

void fun2()
{
   int y;
}

Let's try to compile this thing:

$ g++ main2.cpp -Wall -c -o main2.o
main.cpp: In function ‘void fun1()’:
main.cpp:5:8: warning: unused variable ‘x’ [-Wunused-variable]
    int x;
        ^

There is only a warning for the first unused variable but not for the second. It looks like that some warnings are disabled after the assert macro.

But when I change assert to be one-line statement:

int main()
{
   assert(true==true);
   return 0;
}

then these things:

# 10 "main.cpp" 3 4

are not generated and when I compile the preprocessed file then I get two warnings as expected for two unused variables.

I will get also two warnings when I remove 3 4 from these directives in preprocessed file:

# 10 "main.cpp" 3 4

to 

# 10 "main.cpp"

Why the behaviour of the compiler (g++ 4.9.2) is so different when the condition inside assert is one-line and multiline?

scdmb
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  • Maybe related to [this 10 year old gcc issue](http://stackoverflow.com/a/29314038/1708801) – Shafik Yaghmour Sep 24 '15 at 17:18
  • @ShafikYaghmour This problem is not only about unitialised variables but also about other warnings like comparing signed and unsigned etc. – scdmb Sep 24 '15 at 17:22

0 Answers0