CODEWITHSUNDEEP
c++
Govinda Keshavdas
Govinda Keshavdas

Reputation: 53

Expanding macro within macro

In the following example I expect to see 

 doSomething 
 asserting in "doSomething()"

However i dont see any output on console.

When I use HUTAssert(doSomething()) in main() , I see the expected output, so I expect this has to do with expending a macro within another macro

#include <iostream>
using namespace std;
#define LOPTAssertEnabled 1
#define HUTAssert(expr) ( (expr)? (void(0)) : assertMe(#expr) ) 
#define HAOPAssert(expr) ((isLOPTAssertEnabled())? HUTAssert(#expr) : void(expr))
void assertMe(char const* expr) {
    std::cout <<" asserting in "<<expr; 
}
bool doSomething() {
    std::cout <<" did something "<<std::endl;
    return false;
}

bool isLOPTAssertEnabled() {
    return LOPTAssertEnabled;
}

int main() {
    HAOPAssert(doSomething());
}

Upvotes: 1

Views: 83

Answers (1)

Amadeus
Amadeus

Reputation: 10655

You can easily see what preprocessor is doing. For example, with gcc (g++) you can use the "-E" option to see what preprocessor is producing. In your case, you are getting this:

void assertMe(char const* expr) {
    std::cout <<" asserting in "<<expr;
}

bool doSomething() {
    std::cout <<" did something "<<std::endl;
    return false;
}

bool isLOPTAssertEnabled() {
    return 1;
}

int main() {
    ((isLOPTAssertEnabled())? ( ("doSomething()")? (void(0)) : 
           assertMe("\"doSomething()\"") ) : void(doSomething()));
}

I believe, this is not what your are expecting. However, if you strip off #, which is "stringfying" your token, from HUTAssert(#expr), I believe it will be close to what you are expecting

Upvotes: 4

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