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6.8b — C-style string symbolic constants

C-style string symbolic constants

In the lesson 6.6 -- C-style strings, we discussed how you could create and initialize a C-style string, like this:

C++ also supports a way to create C-style string symbolic constants using pointers:

While these above two programs operate and produce the same results, C++ deals with the memory allocation for these slightly differently.

In the fixed array case, the program allocates memory for a fixed array of length 5, and initializes that memory with the string “Alex\0”. Because memory has been specifically allocated for the array, you’re free to alter the contents of the array.

In the symbolic constant case, how the compiler handles this is implementation defined. What usually happens is that the compiler places the string “Alex\0” into read-only memory somewhere, and then sets the pointer to point to it. Multiple string literals with the same content may point to the same location. Because there’s no guarantee that this memory will be writable, best practice is to make sure the string is const.

Rule: Feel free to use C-style string symbolic constants if you need read-only strings in your program, but always make them const!

std::cout and char pointers

At this point, you may have noticed something interesting about the way std::cout handles pointers of different types.

Consider the following example:

On the author’s machine, this printed:

003AF738
Hello!
Alex

Why did the int array print an address, but the character arrays printed strings?

The answer is that std::cout makes some assumptions about your intent. If you pass it a non-char pointer, it will simply print the contents of that pointer (the address that the pointer is holding). However, if you pass it an object of type char* or const char*, it will assume you’re intending to print a string. Consequently, instead of printing the pointer’s value, it will print the string being pointed to instead!

While this is great 99% of the time, it can lead to unexpected results. Consider the following case:

In this case, the programmer is intending to print the address of variable c. However, &c has type char*, so std::cout tries to print this as a string! On the author’s machine, this printed:

Q╠╠╠╠╜╡4;¿■A

Why did it do this? Well, it assumed &c (which has type char*) was a string. So it printed the ‘Q’, and then kept going. Next in memory was a bunch of garbage. Eventually, it ran into some memory holding a 0 value, which it interpreted as a null terminator, so it stopped.

This case is somewhat unlikely to occur in real-life (as you’re not likely to actually want to print memory addresses), but it is illustrative of how things work under the hood, and how programs can inadvertently go off the rails.

10.1 -- Introduction to dynamic memory allocation
Index
6.8a -- Pointer arithmetic and array indexing

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