Where are string literals stored, and can I modify them?

Asked 2009-04-05T06:40:36.633

I have few questions about string literals in C++.

char *strPtr = "Hello" ;
char strArray[] = "Hello";

Now strPtr and strArray are considered to be string literals. As per my understanding, string literals are stored in read-only memory so we cannot modify their values. We cannot do:

strPtr[2] = 'a';
// or
strArray[2] = 'a';

Both the above statements should be illegal. The compiler should raise errors in both cases, because it keeps string literals in read-only memory, so if we try to modify them, the compiler throws errors.

Also, const objects are considered read-only.

Is it that both string literals and const objects are treated same way? Can I remove constness using const_cast from string literals to change their value?

Where exactly are string literals stored? (In the .data section of program, I assume.)

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2 Answers

Now strPtr and strArray are considered to be string literals.

No, they aren't. String literals are the things you see in your code. For example, the "Hello". strPtr is a pointer to the literal (which is now compiled in the executable). Note that it should be const char *; you cannot legally remove the const per the C standard and expect defined behavior when using it. strArray is an array containing a copy of the literal (compiled in the execuable).

Both the above statements should be illegal. compiler should throw errors in both cases.

No, it shouldn't. The two statements are completely legal. Due to circumstance, the first one is undefined. It would be an error if they were pointers to const chars, though.

As far as I know, string literals may be defined the same way as other literals and constants. However, there are differences:

// These copy from ROM to RAM at run-time:
char myString[] = "hello";
const int myInt = 42;
float myFloats[] = { 3.1, 4.1, 5.9 };

// These copy a pointer to some data in ROM at run-time:
const char *myString2 = "hello";
const float *myFloats2 = { 3.1, 4.1, 5.9 };

char *myString3 = "hello";  // Legal, but...
myString3[0] = 'j';         // Undefined behavior!  (Most likely segfaults.)

My use of ROM and RAM here are general. If the platform is only RAM (e.g. most Nintendo DS programs) then const data may be in RAM. Writes are still undefined, though. The location of const data shouldn't matter for a normal C++ programmer.

answered 2009-04-05T06:46:22.860

The older C and C++ compilers were purely based on low level coding where higher standards of data protection were not available, and they can not even be enforced, typically in C and C++ you can write anything you want..

You can even write a code to access and modify your const pointers as well, if you know how to play with the addresses.

Although C++ does enforce some compile level protection, but there is no protection on runtime. You can certainly access your own stack, and use its values to manipulate any data that came in const pointer as well.

That is the reason C# was invented where little higher level standards are enforced because whatever you access is reference, it is a fixed structure governing all rules of data protection and it has hidden pointer which can not be accessed and nor modified.

The major difference is, C++ can only give you compile time protection, but C# will give you protection even at runtime.

answered 2009-04-05T09:35:16.743

Where are string literals stored, and can I modify them?

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