Despite the name, a constant expression is not a function, but a reference to an object of a certain type. In the context of C++, a constant expression is a template containing a non-static function, and an array or class type that must satisfy constraints to produce a given result. There are numerous types of constant expressions, each with a particular semantic meaning. Constant expressions can be used in contexts that require constant expressions, and their use can be a replacement for template metaprogramming, resulting in faster compilation time. Constant expressions are also an important topic in the world of C/C++ IntelliSense. Constant variables are variables of a certain type that cannot be changed, resulting in the C/C++ IntelliSense aficionado’s dream: a program that works every time.
One of the most notable examples is a constant expression that combines the old school functions of a class or array type with the latest in object oriented design. This is done via a constexpr function, which is required for constant evaluation. The constexpr is responsible for evaluating the aforementioned function, as well as other non-static functions that are relevant to the constant’s lifecycle. Constant expressions may also be used in conjunction with non-constexpr functions, as long as both functions have the same semantic meaning.
The C/C++ compiler is no slouch when it comes to finding and compiling the most relevant information. This is especially true in the case of a constant expression, where reference binding is the norm. The C/C++ compiler is also no slouch when it comes time to identify and fix the myriad problems in the most basic form. The compiler also has the foresight to flag a handful of more esoteric issues.
The best part is that these issues can be easily fixed with a bit of reprogramming. One of the best solutions is to replace template instantiations with constexpr functions. The result is a much clearer and comprehensible program, which is far more likely to achieve the desired results than a collection of ad-hoc functions. The benefits of the new regime also include faster compilation time, and a cleaner program that does not resemble a jumble of drab code. The new regime also allows for an entirely new way to define class static integral fields. This is the best way to define the functionalities of the most common C++ class types.
There are also numerous C/C++ compiler optimizations that can be employed to ensure that a program does not bloat, and is as efficient as possible. For example, constant expressions are only permitted to be used for subobjects of class or array type, as well as objects of a certain type. There are numerous other optimizations as well.