Question

I have a file that has many #define statements such as –

#ifndef UTILITY_H
#define UTILITY_H
#define BUMP 7;
#define WHEEL_DROPS 7;
#define WALL 8;
#define CLIFF_LEFT 9;
#define CLIFF_FRONT_LEFT 10;
#define CLIFF_FRONT_RIGHT 11;
#define CLIFF_RIGHT 12;
#define VIRTUAL_WALL 13;
...
... 
#endif

The list goes on to about 42 different values. I include this file into my other files, but whenever I try to use one of these constants I get errors. For a specific example, I try to do –

Sensor_Packet temp;
temp = robot.getSensorValue(BUMP); //line 54
cout<<temp.values[0]<<endl;

The errors I get are –

main.cpp:54: error: expected ‘)’ before ‘;’ token
main.cpp:54: error: expected primary-expression before ‘)’ token
main.cpp:54: error: expected ‘;’ before ‘)’ token

I don’t see why I am getting these errors because BUMP is already defined. This also happens when I try to use a switch statement with the cases being the defines –

switch(which) {
case BUMP:
    //do stuff
case CLIFF_LEFT:
   //do stuff
}

Is there something I am leaving out about using #define? I thought all I had to do was define a constant and then I could just call it. Any help is appreciated.

Answer 1

Take a closer look at your #defines:

#define BUMP 7;

This tells the preprocessor to replace BUMP with 7;. Note that the macro definition includes the semicolon!

So your code actually looks like this to the compiler:

Sensor_Packet temp;
temp = robot.getSensorValue(7;);
cout<<temp.values[0]<<endl;

// ...

switch(which)
{
case 7;:
    // do stuff
case 9;:
    //do stuff
}

Which are clearly syntax errors. To fix this, remove the semicolons in your #define statements.

But in C++, you should be using const ints or enums for constants instead of #defines. Here are some possible examples:

enum CliffPositions
{
    CLIFF_LEFT  = 9,
    CLIFF_FRONT_LEFT = 10,
    CLIFF_FRONT_RIGHT = 11,
    CLIFF_RIGHT = 12,
};

enum WallType
{
    WALL = 8,
    VIRTUAL_WALL = 13;
}

const int BUMP = 7;
const int WHEEL_DROPS = 7;

// etc ...

This way is preferable because unlike #defines, const ints and enums respect scope and are more type-safe.