Class Functions inlining

Today, while reading a book by Bjarne Stroustrup, I came across an interesting C++ fact that I hadn’t known before.

1. Introduction

Functions defined in a class are implicitly considered as inline, regardless of whether it it is public, protected, or private(access specifier does not affect).

This can be verified through C++ Insights: https://cppinsights.io/s/1a8f7945

source.cpp

class A {
public:
  void funcA() {
  	return;
  }

protected:
  void funcB() {
  	return;
  }
  
private:
  void funcC() {
  	return;
  }
};

translate.cpp

class A  /* size: 1, align: 1 */
{
  
  public: 
  inline void funcA()
  {
    return;
  }
  
  
  protected: 
  inline void funcB()
  {
    return;
  }
  
  
  private: 
  inline void funcC()
  {
    return;
  }
  
};

2. Out-of-Class Definition

If you define a function outside the class declaration, it is not inline by default.

source.cpp

class A {
  void funcA(); // declaration
};


// definition
void A::funcA() {
  return;
}

translate.cpp

class A  /* size: 1, align: 1 */
{
  void funcA();
  
};


void A::funcA()
{
  return;
}

Most of the time, in our project, we separate declarations and definitions into .cpp and .h files. In such case, functions are not inlined by default.

To make out-of-class definition inline, we need to use inline keyword explicitly.

example.cpp

class A {
public:
    inline void funcA();  // Explicitly inline
};

// Now it is inline
inline void A::funcA() {
    return;
}

3. Link-Time Optimization (LTO)

Modern compiler optimize function calls automatically using advanced strategies such as Link-Time Optimization(LTO).

LTO enables cross-translation-unit optimizations, allowing functions defined in .cpp files to be inlined when beneficial.

To enable LTO, we can add a flag -flto to the build command when using GCC/Clang compilers.

4. Conclusion

1. Functions defined inside a class are implicitly inline.
2. Functions defined outside a class are NOT inline unless explicitly marked inline.
3. To enable inlining, either define inside the class or use inline in the definition.