Multi-statement Macros in C++

Question

In C++, is it possible to make a multi-statement macro with nested if statements inside of it like the one below? I\'ve be

Answer 1

The way of the C++:

inline void MATCH_SYMBOL(const Symbol& symbol, const Token& token) {
    /* ... */
    if (something == symbol) {

        if ('-' == symbol) {
        /* ... */
        }
        else if ('-' != symbol) {
        /* ... */
        }
    }
    /* ...other steps... */
}

Answer 2

Note that some of the answers here have a problem.

For example, for a normal statement you can do this:

if (foo)
   function();
else
   otherstuff();

If you followed some of the suggestions here, but if replace function with a macro, it might expand to:

if (foo)
   if (something) { /* ... */ }
   else           { /* ... */ }; // <-- note evil semicolon!
else
   otherstuff();

So a common (ugly) hack that people do to avoid this is:

#define MATCH_SYMBOL(symbol, token)    \
    do                                 \
    {                                  \
       if(something == symbol)         \
       {                               \
          if( symbol == '-')           \
          {                            \
          }                            \
          else if (symbol != '-')      \
          {                            \
          }                            \
          other steps;                 \
       }                               \
    }                                  \
    while (0) // no semicolon here

This is so that the "statement" MATCH_SYMBOL(a, b) can end with a semicolon just like a normal statement. You also have braces around the multiple statements.

If you think nobody's crazy enough to use this technique, think again. It's very common in the Linux kernel, for example.

Answer 3

One can also define macro function and implement the function than

#define MATCH_SYMBOL( symbol, token) match_symbol(symbol,token)

Answer 4

If you're using C++ you should avoid using macros altogether. They are not type-safe, they're not namespace-aware, they're hard to debug and just they're plain messy.

If you need a type-independent function, use templates:

template <typename T>
bool match_symbol(T symbol, T token) {
    if(something == symbol){
        if( symbol == '-'){
        }else if (symbol != '-'){
        }
    ...

or if the parameters can be different types:

template <typename T, typename V>
bool match_symbol(T symbol, V token) {
    if(something == symbol){
        if( symbol == '-'){
        }else if (symbol != '-'){
        }
    ...

Answer 5

You need to have a backslash (\) at the end of all the lines in the macro but the last one.

Answer 6

also, see if you can replace the macro with a function.

?
MATCH_SYMBOL(Sym const & symbol, Tok const & token)
{
    ...
}