8.6.2. Nonconstant Pointer to Constant Data

A nonconstant pointer to constant data is a pointer that can be modified to point to any data item of the appropriate type, but the data to which it points cannot be modified through that pointer. Such a pointer might be used to receive a built-in array argument to a function that should be allowed to read the elements, but not modify them. Any attempt to modify the data in the function results in a compilation error. The declaration for such a pointer places const to the left of the pointer’s type, as in

The declaration is read from right to left as “countPtr is a pointer to an integer constant” or more precisely, “countPtr is a non-constant pointer to an integer constant.”

Figure 8.10 demonstrates GNU C++’s compilation error message produced when attempting to compile a function that receives a nonconstant pointer to constant data, then tries to use that pointer to modify the data.

When a function is called with a built-in array as an argument, its contents are effectively passed by reference because the built-in array’s name is implicitly convertible to the address of the built-in array’s first element. However, by default, objects such as arrays and vectors are passed by value—a copy of the entire object is passed. This requires the execution-time overhead of making a copy of each data item in the object and storing it on the function call stack. When a pointer to an object is passed, only a copy of the address of the object must be made—the object itself is not copied.