C++ "const" keyword explanation

Question

When reading tutorials and code written in C++, I often stumble over the const keyword.

I see that it is used like the following:

const int x = 5; 

I know that this means that x is a constant variable and probably stored in read-only memory.

But what are

void myfunc( const char x ); 

and

int myfunc( ) const; 

?

Answer 1

void myfunc(const char x); 

This means that the parameter x is a char whose value cannot be changed inside the function. For example:

void myfunc(const char x) {   char y = x;  // OK   x = y;       // failure - x is `const` } 

For the last one:

int myfunc() const; 

This is illegal unless it's inside a class declaration - const member functions prevent modification of any class member - const nonmember functions cannot be used. in this case the definition would be something like:

int myclass::myfunc() const {   // do stuff that leaves members unchanged } 

If you have specific class members that need to be modifiable in const member functions, you can declare them mutable. An example would be a member lock_guard that makes the class's const and non-const member functions threadsafe, but must change during its own internal operation.

Answer 2

The first function example is more-or-less meaningless. More interesting one would be:

void myfunc( const char *x ); 

This tells the compiler that the contents of *x won't be modified. That is, within myfunc() you can't do something like:

strcpy(x, "foo"); 

The second example, on a C++ member function, means that the contents of the object won't be changed by the call.

So given:

class {   int x;   void myfunc() const; } 

someobj.myfunc() is not allowed to modify anything like:

x = 3;