macro

Code:

---

#define PRINT(STR,VAR) \
        cout << STR " = "  VAR << endl
int i = 10;
PRINT("i;", i);

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HI, how come this macro doen't work?

1. Don't use the Macro.

2. You forgot the semicolon, you forgot <<

#define PRINT(STR,VAR) \
cout << STR << " = " << VAR << endl;

use a template function instead

Code:

---

template<typename T>
void PRINT(const std::string& name, const T value)
{
    std::cout << name << " = " <<  value << std::endl;
}

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Quote:

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Originally Posted by souldog

1. Don't use the Macro.

2. You forgot the semicolon, you forgot <<

#define PRINT(STR,VAR) \
cout << STR << " = " << VAR << endl;

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it doens't work; I copied this example in TC++, chapter "the C in c++";
at the moment I'd like learn something about macro...

Well it is impossible to know without you giving a hint as to what is not working.


Look at it this way: What you are now learning is why you should never use macros to do things like this.

Macros have there uses, this is not one of them

Quote:

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Originally Posted by mickey0

HI, how come this macro doen't work?

---

Well, what is the output that you expect to get?

I expect
i: 10

The real problem is that is doesn't compile.

Quote:

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Originally Posted by mickey0

I expect
i: 10

The real problem is that is doesn't compile.

---

Try this:

Code:

---

#include <iostream>

#define PRINT(STR,VAR) \
                std::cout << #STR << " " <<  VAR << std::endl

int main()
{
        int i = 10;
        PRINT(i;, i);

        return 0;
}

---

The trick is where I put the # in front of STR in the macro.

nothing to do:
However the errors without sense for me) are:

Code:

---

error C2143: syntax error : missing ';' before '<<'
error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
error C2014: preprocessor command must start as first nonwhite space

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and other error in all the program (that I haven't if I comment that macro)...

This works perfectly for me.

Code:

---

#include<iostream>
#define PRINT(STR,VAR) \
std::cout<<STR<<"="<<VAR<<std::endl;
int main()
{
PRINT("Variable",10)
return 0;
}

---

I tried with VC++6.0 and VS2005

Bharani

Quote:

---

Originally Posted by souldog

Well it is impossible to know without you giving a hint as to what is not working.


Look at it this way: What you are now learning is why you should never use macros to do things like this.

Macros have there uses, this is not one of them

---

Why should not we use macro here?

Bharani

Quote:

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Why should not we use macro here?

---

Read Stroustrup's answer to the FAQ: So, what's wrong with using macros?

Thank you, laserlight! That's a nice piece of information :)

One reason why I hate macro 'functions' is that it is source code created at compile time and as such it can be very difficult to figure out what code you're actually running!

On a related note, I think template meta-programming potentially suffers same problems as it is also source code created at compile time. A complicated template meta-program can be difficult to decypher.

just replace the macro name with the text you have defined after the macro(that's all the compiler does) and you'll see what's wrong eg.

#define PRINT(STR,VAR) \
cout << STR " = " VAR << endl
int i = 10;
PRINT("i;", i);


replaced by:

cout << "i;" " = " 10 << endl

you can see that there should be << inbetween i; and = and there should a semi-colon after endl. If you MUST use a macro use:

#define PRINT(STR, VAR) \
std::cout << STR << " = " << (VAR) << std::endl;

also, you should include the namespace std:: before cout and endl unless you are sure "using namespace std;" has been called before the macro.