CODEWITHSUNDEEP
c++
edo101
edo101

Reputation: 639

Undeclared Identifier error using simple macro C++

Check this out: 

#include <iostream>  //input outut like cout, cin
#include <string>  //strings
#include <cstdlib>  //includes random num generators and basic c++ functions 
#include <limits> //functions for min and max for data types 
#include <vector>
#include <numeric> //sequences of values
#include <cmath> //math functions
#include <sstream> //string stream
#include <ctime> //time
#include <algorithm> //includes sort
#include <fstream>  //alllows ofstream and ifstream for some reason? unicode vs ansi? 
#include "Shape.h"
#include "Circle.h"
#include <functional>  //allows you to use function poitner? <function>   

#define PI 3.14159 //anywhere you see PI, it will be replaced with the number to its right 
    #define AREA_CIRC (radius) (PI * pow(radius, 2))

    int main(){

    cout << "Circle Area " << AREA_CIRC(5) << endl;

    }

Whenever I run this code, it gives me this error:

Error C2065 'radius': undeclared identifier Derektut

Why? declaring int radius in the macro definition makes no difference

Upvotes: 3

Views: 1803

Answers (3)

user10957435
user10957435

Reputation:

The below was mostly written before you made your edit, so I will suggest removing the space between AREA_CIRC and (radius) and see if that helps (it worked for me).

Worth noting that the rest still mostly applies, except maybe the part which suggested getting rid of the type declaration.

Macros don't use type declarations. They are pure text substitutions (which makes them somewhat unsafe to use). Thus, int here is unnecessary:

#include <iostream>
#include <cmath>
using namespace std;

#define PI 3.14159 //anywhere you see PI, it will be replaced with the number to its right 
#define AREA_CIRC(radius) (PI * pow(radius, 2))

int main(){

cout << "Circle Area " << AREA_CIRC(5) << endl;

}

That should work, though especially if you're using macros, I don't recommend using namespace std even though I've used it here quickly to create an example.

Ultimately, it'd be better to use a constexpr function instead:

constexpr float AREA_CIRC (int radius) { // double will work here as well, if desired
    return (PI * pow(radius, 2));
}

Also, excellent suggestion by @passing_through, you can also change PI to a constexpr as well:

constexpr double PI = 3.14159;

Upvotes: 2

user1196549
user1196549

Reputation:

Your macro expands as

cout << "Circle Area " << (radius) (PI * pow(radius, 2))(5) << endl;

not the way you expected. @passing_through gave you the cure.

Upvotes: 2

passing_through
passing_through

Reputation: 1931

#define AREA_CIRC (radius) (PI * pow(radius, 2)) means

replace all AREA_CIRC with (radius) (PI * pow(radius, 2))

As a result, you just get plain text substitution including radius which really is an unknown identifier. What you probably meant was a function-like macro:

#define AREA_CIRC(radius) (PI * pow(radius, 2))

Just remove the space between the macro name and its opening bracket.

Upvotes: 8

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