## Three types of Constants that exist in C++

In a previous posting, I explained that I am able to write C++ programs, that test some aspect of how the CPU performs simplistic computations, even though C++ is a high-level language, which its compiler tries to optimize, before generating a machine-language program, the latter of which is called the run-time.

As I pointed out in that posting, there exists a danger that the computation may not take place the way in which the code reads to plain inspection, more specifically, in the fact that certain computations will be performed at compile-time, instead of at run-time. In reality, both the compile-time and the run-time computations as such still take place on the CPU, but in addition, a compiler can parse and then examine larger pieces of code, while a CPU needs to produce output, usually just based on the contents of a few registers – in that case, based on the content of up to two input-registers.

I feel that in contrast with the example which I wrote will work, I should actually provide an example, which will not work as expected. The following example is based on the fact that in C++, there exist three basic types of constants:

1. Literals,
2. Declared constants,
3. Constant Expressions.

Constant expressions are any expressions, the parameters of which are never variables, always operators, functions, or other constants. While they can increase the efficiency with which the program works, the fact about constant expressions which needs to be remembered, is that by definition, the compiler will compute their values at compile-time, so that the run-time can make use of them. The example below is an example in which this happens, and which exists as an antithesis of the earlier example:


// This code exemplifies how certain types of
// computations will be performed by the compiler,
// and not by the compiled program at run-time.

// It uses 3 types of constants that exist in C++ .

#include <cstdio>

int main () {

// A variable, initialized with a literal
double x = 0;

// A declared constant
const double Pi = 3.141592653589;

// A constant expression, being assigned to the variable
x = ( Pi / 6 );

printf("This is approximately Pi / 6 : %1.12f\n", x);

return 0;
}





dirk@Plato:~/Programs$./simp_const This is approximately Pi / 6 : 0.523598775598 dirk@Plato:~/Programs$




If this was practical code, there would be no problem with it, purely because it outputs the correct result. But if this example was used to test anything about how the run-time behaves, its innocent Math will suffer from one main problem:

When finally assigning a computed value to the double-precision floating-point variable ‘x’ , the value on the right-hand side of the assignment operator, the ‘=’ , is computed by the compiler, before a machine-language program has been generated, because all of its terms are either literals or declared constants.

This program really only tests, how well the ‘printf()’ function is able to print a double-precision value, when instructed exactly in what format to do so. That value is contained in the machine-code of the run-time, and not computed by it.

BTW, There exists a type of object in C, called a Compound Literal, which is written as an Initializer List, preceded by a data-type, as if to type-cast that initializer list to that data-type. But, because all manner of syntax that involves initializer lists is distinctly C and not C++ , I’ve never given much attention to compound literals.

(Edit 11/13/2017 : )

In other words, if the code above contained a term of the form

( 1 / ( 1 / Pi ))

Then chances are high, that because this also involves a constant expression, the compiler may simplify it to

( Pi )

Without giving the programmer any opportunity, to test the ability of the CPU, to compute any reciprocals.

Dirk