# Literal with number and unit in an algebraic expression as argument

• A+
Category：Languages

How to design a type that can take an expression in the form of an algebraic expression with numbers and units, where `Speed` is the type, `23m / 10s` could be an expression?

Example:

``change_speed(Speed s);    // better: the meaning of s is specified // ... change_speed(2.3);        // error: no unit change_speed(23m / 10s);  // meters per second ``

EDIT This question raised the question if a literal in the form of `23m` is possible at all.

EDIT the reference to the source has been removed from question for clarity. It can be found here

What you have to do is design types that when the expression is evaluated it yields the desired type. With

``change_speed(23m / 10s); ``

We need `23m` and `10s` to each yield a type that when you divide them together gives you a `Speed`. If we have a

``struct Meter {     Meter(double value) : value(value) {}     double value; };  struct Second {     Second(double value) : value(value) {}     double value; }  Speeds operator /(const Meter& m, const Second& s) {     return Speed(m.value / s.value); } ``

Then we just need to define literal operators for `Meter` and `Second` like

``Meter operator "" _m(double value) { return Meter(value); }  Second operator "" _s(double value) { return Second(value); } ``

we have to use `_m` and `_s` to make these valid user defined literals. All literal not from the standard have to start with a `_`.

Then

``change_speed(23_m / 10_s) ``

becomes

``change_speed(Meter(23) / Second(10)) ``

which becomes

``change_speed(Speed(2.3)) ``