When i compile the below program following error is generated by the compiler.
example.cpp:12:13: error: invalid operands to binary expression ('const char *' and 'const char *')
cout << JMP(to_string(10)) << endl;
^~~~~~~~~~~~~~~~~~
example.cpp:6:34: note: expanded from macro 'JMP'
#define JMP(add) "DEFAULT is : " + DEFAULT + " JMP is : " + add
~~~~~~~~~~~~~~~ ^ ~~~~~~~
#include <iostream>
#include <string>
#define DEFAULT "00000"
#define JMP(add) "DEFAULT is : " + DEFAULT + " JMP is : " + add
using namespace std;
int main()
{
cout << JMP(to_string(10)) << endl;
return 0;
}
Whereas the below program compiles properly
#include <iostream>
#include <string>
#define DEFAULT "00000"
#define JMP(add) "JMP is : " + add + "DEFAULT is : " + DEFAULT
using namespace std;
int main()
{
cout << JMP(to_string(10)) << endl;
return 0;
}
Why the order of argument present in the macro body matters ?
Try to get rid of the +
to concatenate char array literals:
#define JMP(add) "DEFAULT is : " DEFAULT " JMP is : " add
NOTE: Since add
will expand to a std::string
value ( to_string(10)
) from your sample, this won't work either. You'll need to call the macro like this:
cout << JMP("10") << endl;
An alternate solution would be making the parts std::string
instances:
#define JMP(add) std::string("DEFAULT is : " DEFAULT " JMP is : ") + add
The error is telling you that the operands given to the binary expression (ie + operator
) are not of expected types. This operator is expecting a const string &
(or a string &
using C++ 11) for at least one of its operands. That plus left to right evaluation is why it works when you switch the order.
cout << to_string(10) + " is the JUMP" << endl; // having valid operands, + operator returns a string object
cout << "JUMP is : " + to_string(10) << endl; // same here
cout << "DEFAULT is : " + "00000" << endl; // no bueno: both operands are const char pointers
Provided that you have a const string &
as a starter* , you can keep concatenating const char *
s all day long:
cout << "JUMP is : " + to_string(10) + " DEFAULT is : " + "00000" + "game is " + "concentration, " + "category is " + "..." << endl;
So, this actually is not about order of macro arguments but about strings, char pointers, concatenation, associativity, and operators.
*as in a sports game.
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