Euri Pinhollow Euri Pinhollow - 1 year ago 54
C Question

Deterministic way of saying "promote everything to floating before calculation" in C++

Given that I'd prefer to keep numbers in my program as

s or whatever integral, what is the most convenient way of doing an arbitrary arithmetic with floating point equivalents of those numbers?

Say, I have

int a,b,c,d;
double x;

And I want to write


without turning the expression into mess by putting conversions everywhere in parsed operator tree leafs like


Is it doable with C-style macro (recursive or not)? Should it be done with new class and overloaded operators?

Answer Source

This is a pretty complex expression. Better give it a name:

double complex_expression(double a, double b, double c, double d) {
  return a/b/c/d+c/d+a;

Now when you call that with integer arguments, since the parameters are of type double the arguments get converted to double using the usual arithmetic conversions:

int a,b,c,d;
// Init them somehow
double x = complex_expression(a,b,c,d);

With a C++11 lambda ...

int a,b,c,d;
// Init them somehow
double x = [](double a, double b, double c, double d) {
  return a/b/c/d+c/d+a; }(a,b,c,d);

... works, but IMO somehow looks clumsy.

Slightly better?

double x = [a = (double)a, b = (double)b, c = (double)c, d = (double) d] {
    return a/b/c/d+c/d+a; }();

Oh, and if you're in for some macro fun:

#define SPLICE_2(l,r) l##r
#define SPLICE_1(l,r) SPLICE_2(l,r)
#define SPLICE(l,r) SPLICE_1(l,r)

#define TREAT_AS(type, name) name = static_cast<type>(name)

#define TREAT_ALL_AS_HELPER_0(type)
#define TREAT_ALL_AS_HELPER_1(type, name)  TREAT_AS(type, name)
#define TREAT_ALL_AS_HELPER_2(type, name, ...) TREAT_AS(type, name), TREAT_ALL_AS_HELPER_1(type, __VA_ARGS__)
#define TREAT_ALL_AS_HELPER_3(type, name, ...) TREAT_AS(type, name), TREAT_ALL_AS_HELPER_2(type, __VA_ARGS__)
#define TREAT_ALL_AS_HELPER_4(type, name, ...) TREAT_AS(type, name), TREAT_ALL_AS_HELPER_3(type, __VA_ARGS__)
#define TREAT_ALL_AS_HELPER_5(type, name, ...) TREAT_AS(type, name), TREAT_ALL_AS_HELPER_4(type, __VA_ARGS__)
// expand as you will

#define TREAT_ALL_AS(type, count, ...) SPLICE(TREAT_ALL_AS_HELPER_, count)(type, __VA_ARGS__)

Now use as

double x = [TREAT_ALL_AS(double, 4, a, b, c, d)] {
  return a/b/c/d+c/d+a; }();

You can also automatically count the number of variadic macro arguments.

But to be honest, IMO its best to just write a named function.

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