Programming Forums > Application Development > C++

aznluvsmc · Mar 31st, 2006, 7:48 PM

Hi,

I'm learning about namespaces right now and I was wondering if #define is allowed inside a namespace? If so, can I access the constant with a using declaration?

Here's an example:

(Toggle Plain Text)

#include <iostream>

namespace my_name {
     #define LOOP 10
}

using std::cout;
using std::endl;

int main() {
    for(int i = 0; i < my_name::LOOP; i++)
            cout << i << endl;

    return 0;

}

I know this code will not compile on an AIX machine. For some reason the compiler thinks that my_name::LOOP which is the value 10 is a string.

DaWei · Mar 31st, 2006, 9:34 PM

#define is a preprocessing statement. That means the substitution is made before the compiler sees the source. Look at your code in view of that textual substitution.

grumpy · Apr 1st, 2006, 4:15 AM

Your source code is fed to the preprocessor (and text substitutions made) before the compiler (the program that understands C++ constructs, such as namespace) sees the code. #define therefore does not respect namespaces. Placing a macro in a namespace makes no difference to its effect, and macros do not exist in namespaces.

In your example. the preprocessor sees my_name::LOOP as a potential name which is something distinct from your macro LOOP.

One other consequence of the required behaviour of the preprocessor is that;

(Toggle Plain Text)

#include <iostream>

#define LOOP 10

namespace my_name
{

}

int main()
{
    std::cout << LOOP << '\n';
}

is exactly equivalent to;

(Toggle Plain Text)

#include <iostream>

namespace my_name
{
#define LOOP 10
}

int main()
{
    std::cout << LOOP << '\n';
}

aznluvsmc · Apr 1st, 2006, 8:40 PM

Yes, now I see it. Thanks for clearing that up.

Mar 31st, 2006, 7:48 PM	#1
aznluvsmc Hobbyist Programmer Join Date: Aug 2005 Posts: 137 Rep Power: 4	Are #define allowed inside namespaces? Hi, I'm learning about namespaces right now and I was wondering if #define is allowed inside a namespace? If so, can I access the constant with a using declaration? Here's an example: (Toggle Plain Text) #include <iostream> namespace my_name { #define LOOP 10 } using std::cout; using std::endl; int main() { for(int i = 0; i < my_name::LOOP; i++) cout << i << endl; return 0; } #include <iostream> namespace my_name { #define LOOP 10 } using std::cout; using std::endl; int main() { for(int i = 0; i < my_name::LOOP; i++) cout << i << endl; return 0; } I know this code will not compile on an AIX machine. For some reason the compiler thinks that my_name::LOOP which is the value 10 is a string.

Mar 31st, 2006, 9:34 PM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	#define is a preprocessing statement. That means the substitution is made before the compiler sees the source. Look at your code in view of that textual substitution. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Apr 1st, 2006, 4:15 AM	#3
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	Your source code is fed to the preprocessor (and text substitutions made) before the compiler (the program that understands C++ constructs, such as namespace) sees the code. #define therefore does not respect namespaces. Placing a macro in a namespace makes no difference to its effect, and macros do not exist in namespaces. In your example. the preprocessor sees my_name::LOOP as a potential name which is something distinct from your macro LOOP. One other consequence of the required behaviour of the preprocessor is that; (Toggle Plain Text) #include <iostream> #define LOOP 10 namespace my_name { } int main() { std::cout << LOOP << '\n'; } #include <iostream> #define LOOP 10 namespace my_name { } int main() { std::cout << LOOP << '\n'; } is exactly equivalent to; (Toggle Plain Text) #include <iostream> namespace my_name { #define LOOP 10 } int main() { std::cout << LOOP << '\n'; } #include <iostream> namespace my_name { #define LOOP 10 } int main() { std::cout << LOOP << '\n'; }

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Apr 1st, 2006, 8:40 PM	#4
aznluvsmc Hobbyist Programmer Join Date: Aug 2005 Posts: 137 Rep Power: 4	Yes, now I see it. Thanks for clearing that up.

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Programming Forums > Application Development > C++

Are #define allowed inside namespaces?