macro

[mickey0]

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

HI, how come this macro doen't work?

[souldog]

1. Don't use the Macro.

2. You forgot the semicolon, you forgot <<

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

[souldog]

use a template function instead


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

[mickey0]

1. Don't use the Macro.

2. You forgot the semicolon, you forgot <<

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

[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

[angelorohit]

HI, how come this macro doen't work?Well, what is the output that you expect to get?

[mickey0]

I expect
i: 10

The real problem is that is doesn't compile.

[angelorohit]

I expect
i: 10

The real problem is that is doesn't compile.Try this:

#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.

[mickey0]

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

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

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

[ajbharani]

This works perfectly for me.

#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

[ajbharani]

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

[laserlight]

Why should not we use macro here?
Read Stroustrup's answer to the FAQ: So, what's wrong with using macros? (http://www.research.att.com/~bs/bs_faq2.html#macro)

[ajbharani]

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

[JohnW@Wessex]

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.

[staticVoid]

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.

[mickey0]

I doens't still work; my code now is:

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

int main (int argc, char argv[]) {
int i = 10;
PRINT("var", i);
return EXIT_SUCCESS;
}


error C2143: syntax error : missing ';' before '<<'
error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
error C2371: 'std::cout' : redefinition; different basic types
see declaration of 'std::cout'
error C2017: illegal escape sequence //this error is on PRINT("var",i);

[ajbharani]

#define PRINT(STR, VAR) \

Please eliminate the blank space after the '\' and re-compile. It works then.

Bharani

[staticVoid]

I doens't still work; my code now is:

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

int main (int argc, char argv[]) {
int i = 10;
PRINT("var", i);
return EXIT_SUCCESS;
}


error C2143: syntax error : missing ';' before '<<'
error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
error C2371: 'std::cout' : redefinition; different basic types
see declaration of 'std::cout'
error C2017: illegal escape sequence //this error is on PRINT("var",i);



warning: enclose VAR in () : (VAR), If you have an expression like:

PRINT("1 shift left 5: ", 1 << 5)

this will produce:

std::cout<<"1 shift left 5"<<" = "<< 1 << 5 <<std::endl;

you can see the problem. this is another reason not to use macros.

[mickey0]

#define PRINT(STR, VAR) \

Please eliminate the blank space after the '\' and re-compile. It works then.
Bharani
THANKS!!!!!!!
Now it works even the version on the TC++ that is:

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

THis above compile and work!
It was the white space after '\'
I didn't know that!

Thanks.