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#pragma

 
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#pragma
Suryakant
7 Jul, 2008 - 04:49 AM
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what is
#if
#endif
#elseif ????
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gabehabe
RE: #pragma
7 Jul, 2008 - 04:56 AM
Post #2

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Preprocessor directives. Basically they are commands which we give to the compiler.

The most typical uses are:
#pragma once tells the compiler to only include a specific file once
#ifndef MYHEADER_H_INCLUDED if the file has not been defined, then we need to do something
#define MYHEADER_H_INCLUDED will define the header class
#endif ends the if statement
The most common way to define a header file is by using ifndef like so:
cpp
#ifndef MYHEADER_H_INCLUDED
#define MYHEADER_H_INCLUDED
// declarations go here, eg:
class CmyClass
{
    private:
        int a;
    public:
        CmyClass();
        ~CmyClass();
};
#endif

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captainhampton
RE: #pragma
7 Jul, 2008 - 11:18 AM
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Here is a quick run down/tutorial (very good) on some other preprocessor directives:
http://www.cplusplus.com/doc/tutorial/preprocessor.html
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polymath
RE: #pragma
7 Jul, 2008 - 04:45 PM
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Anything that starts with a # is a preprocessor directive. Basically, it's a very easy way to include code and define variables. Though they don't make it into the machine code, the compiler carries them out and changes the file before use. For example, if you use #define PI 3.14, then PI would not be treated as an identifier by the compiler, but each instance of PI would be replaced by 3.14

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Tom9729
RE: #pragma
7 Jul, 2008 - 05:52 PM
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The preprocessor runs through your code before you compile and replaces preprocessor macros (things that start with #) with code.

For example, the snippet below would only insert the hypothetical debug code if DEBUG is defined (which you can do with #define DEBUG or by supplying -DDEBUG as a command line argument to the compiler (supported on GCC, not sure about how many others).
cpp

#ifdef DEBUG

// Put some debug code here.

#endif


It's basically an easy way to toggle different parts of your code on/off, and to insert values and bits of code without having to manually write them in.

More on it here. smile.gif
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