Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Type induction in function templates

As a simple but very useful example, consider the following: 

//: :arraySize.h
// Uses template type induction to 
// discover the size of an array
#ifndef ARRAYSIZE_H
#define ARRAYSIZE_H

template<typename T, int size>
int asz(T (&)[size]) { return size; }
#endif // ARRAYSIZE_H ///:~

This actually figures out the size of an array as a compile-time constant value, without using any sizeof( ) operations! Thus you can have a much more succinct way to calculate the size of an array at compile time: 

//: C19:ArraySize.cpp
// The return value of the template function
// asz() is a compile-time constant
#include "../arraySize.h"

int main() {
  int a[12], b[20];
  const int sz1 = asz(a);
  const int sz2 = asz(b);
  int c[sz1], d[sz2];
} ///:~

Of course, just making a variable of a built-in type a const does not guarantee it’s actually a compile-time constant, but if it’s used to define the size of an array (as it is in the last line of main( )), then it must be a compile-time constant.

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