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C++ Functions Explained

Instructor Rainer Grimm distills the C++ Functions rules and best practices from the Core Guidelines.
This chapter is from the book

Software developers master complexity by dividing complex tasks into smaller units. After the small units are addressed, they put the smaller units together to master the complex task. A function is a typical unit and, therefore, the basic building block for a program. Functions are “the most critical part in most interfaces . . .” (C++ Core Guidelines about functions).

The C++ Core Guidelines have about forty rules for functions. They provide valuable information on the definition of functions, how you should pass the arguments (e.g., by copy or by reference), and what that means for the ownership semantics. They also state rules about the semantics of the return value and other functions such as lambdas. Let’s dive into them.

Function definitions

Presumably, the most important principle for good software is good names. This principle is often ignored and holds true in particular for functions.

Good names

The C++ Core Guidelines dedicate the first three rules to good names: “F.1: ‘Package’ meaningful operations as carefully named functions,” “F.2: A function should perform a single logical operation,” and “F.3: Keep functions short and simple.”

Let me start with a short anecdote. A few years ago, a software developer asked me, “How should I call my function?” I told him to give the function a name such as verbObject. In case of a member function, a verb may be fine because the function already operates on an object. The verb stands for the operation that is performed on the object. The software developer replied that this is not possible; the function must be called getTimeAndAddToPhonebook or just processData because the functions perform more than one job (single-responsibility principle). When you don’t find a meaningful name for your function (F.1), that’s a strong indication that your function does more than one logical operation (F.2) and that your function isn’t short and simple (F.3). A function is too long if it does not fit on a screen. A screen means roughly 60 lines by 140 characters, but your measure may differ. Now you should identify the operations of the function and package these operations into carefully named functions.

The guidelines present an example of a bad function:

void read_and_print() { // bad
   int x;
   std::cin >> x;
   // check for errors
   std::cout << x << '\n';

The function read_and_print is bad for many reasons. The function is tied to a specific input and output and cannot be used in a different context. Refactoring the function into two functions solves these issues and makes it easier to test and to maintain:

int read(std::istream& is) { // better
   int x;
   is >> x;
   // check for errors
   return x;

void print(std::ostream& os, int x) {
   os << x << '\n';

A constexpr function is a function that has the potential to run at compile time. When you invoke a constexpr function within a constant expression, or you take the result of a constexpr with a constexpr variable, it runs at compile time. You can invoke a constexpr function with arguments that can be evaluated only at run time, too. constexpr functions are implicit inline.

The result of constexpr evaluated at compile time is stored in the ROM (read-only memory). Performance is, therefore, the first big benefit of a constexpr function. The second is that constexpr functions evaluated at compile time are const and, therefore, thread safe.

Finally, a result of the calculation is made available at run time as a constant in ROM.

// constexpr.cpp

constexpr auto gcd(int a, int b) {
   while (b != 0) {
      auto t = b;
      b = a % b;
      a = t;
   return a;


int main() {

   constexpr int i = gcd(11, 121); // (1)

   int a = 11;
   int b = 121;
   int j = gcd(a, b);             // (2)


Figure 4.1 shows the output of Compiler Explorer and depicts the assembly code generated by the compiler for this function. I used the Microsoft Visual Studio Compiler 19.22 without optimization.

Figure 4.1

Figure 4.1 Assembler instructions to the program constexpr.cpp

Based on the colors, you can see that (1) in the source code corresponds to line 35 in the assembler instructions and (2) in the source code corresponds to lines 38–41 in the assembler instructions. The call constexpr int i = gcd(11, 121); boils down to the value 11, but the call int j = gcd(a, b); results in a function call.

By declaring a function as noexcept, you reduce the number of alternative control paths; therefore, noexcept is a valuable hint to the optimizer. Even if your function can throw, noexcept often makes much sense. noexcept means in this case: I don’t care. The reason may be that you have no way to react to an exception. Therefore, the only way to deal with exceptions is to invoke std::terminate(). This noexcept declaration is also a piece of valuable information for the reader of your code.

The next function just crashes if it runs out of memory.

std::vector<std::string> collect(std::istream& is) noexcept {
   std::vector<std::string> res;
   for (std::string s; is >> s;) {
  return res;

The following types of functions should never throw: destructors (see the section Failing Destructor in Chapter 5), swap functions, move operations, and default constructors.

Pure functions are functions that always return the same result when given the same arguments. This property is also called referential transparency. Pure functions behave like infinite big lookup tables.

The function template square is a pure function:

template<class T>
auto square(T t) {
   return t * t;

Conversely, impure functions are functions such as random() or time(), which can return a different result from call to call. To put it another way, functions that interact with state outside the function body are impure.

Pure functions have a few very interesting properties. You should, therefore, prefer pure functions, if possible.

Pure functions can

  • Be tested in isolation

  • Be verified or refactorized in isolation

  • Cache their result

  • Automatically be reordered or be executed on other threads

Pure functions are also often called mathematical functions. Functions in C++ are by default not pure such as in the pure functional programming language Haskell. Using pure functions is based on the discipline of the programmer. constexpr functions are pure when evaluated at compile time. Template metaprogramming is a pure functional language embedded in the imperative language C++.

Chapter 13, Templates and Generic Programming, gives a concise introduction to programming at compile time, including template metaprogramming.

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