69 lines
1.8 KiB
Markdown
69 lines
1.8 KiB
Markdown
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# Special operators '#' and '##' inside #define definitions
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Inside macro definitions, two special operators can be used:
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- a single hash symbol '#' before the name of a macro parameter turns the contents of that parameter into a string; it is allowed only in function macros;
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- a double hash symbol '##' between two words (tokens) combines them, and if the token is a macro parameter, then its value is substituted, but if the token is a macro name, it is substituted as is, without expanding the macro; if as a result of "gluing" another macro name is obtained, it is expanded;
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In the examples in this book, we often used the following macro:
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```
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#define PRT(A) Print(#A, "=", (A))
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```
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It calls the Print function, in which the passed expression is displayed as a string thanks to #A, and after the sign "equal", the actual value of A is printed.
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To demonstrate '##', let's consider another macro:
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```
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#define COMBINE(A,B,X) A##B(X)
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```
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With it, we can actually generate a call to the SQN macro defined above:
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```
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Print(COMBINE(SQ,N,2)); // 4
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```
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The literals SQ and N are concatenated, after which the macro SQN expands to ((2)*(2)) and produces the result 4.
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The following macro allows you to create a variable definition in code by generating its name given the parameters of the macro:
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```
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#define VAR(TYPE,N) TYPE var##N = N
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```
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Then the line of code:
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```
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VAR(int, 3);
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```
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is equivalent to the following:
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```
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int var3 = 3;
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```
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Concatenation of tokens allows the implementation of a loop shorthand over the array elements using a macro.
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```
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#define for_each(I, A) for(int I = 0, max_##I = ArraySize(A); I < max_##I; ++I)
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// describe and somehow fill in the array x
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double x[];
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// ...
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// implement loop through the array
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for_each(i, x)
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{
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x[i] = i * i;
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}
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```
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