Conversion from string constant, pointers in c++

Question

After reading several answers I have corrected my code to as follows;

int main()
{
    // a pointer to char is initialized with a string literal

    char Buffer[100];

    cout << "Enter an initial string: " << endl;
    cin >> Buffer;
    cout << "Original content of Buffer:--> " << Buffer << endl;


    cout << "Enter a sentence: " << endl;
    cin >> Buffer;



    gets(Buffer);
    cout << "Now the Buffer contains:--> " << Buffer << endl;

    return 0;
}

I know longer have the warning code, but now the program doesnt execute as I would like. The last part does not output my new sentance.

I know people mentioned not to use gets, but I tried using getline, obviously I cant use it as a direct replacement so I was a bit lost.

Any suggestions

Answer 1

You cannot read into a memory which contains string constant. Often those string constants are stored in read-only memory and even if not, they can share the constants so you would override one string for all parts of your code.

You need to copy the string into some buffer and then do whatever you want. For example:

const char *myOrigBuffer = "Dummy string";
char buffer[1024];
strcpy(buff, myOrigBuffer);
....
gets(buff);

Answer 2

You cannot modify string literral. Your way of coding is too much "C style". If the original buffer content doesn't matter and you must use gets(), don't initialize your buffer :

char Buffer[100];
cout << "Enter a sentence: " << endl;

gets(Buffer);
cout << "Now the Buffer contains:--> " << endl;
cout << Buffer << endl;

Don't forget that if you input more than 100 characters (as the size of the buffer), this will also crash.

As gets is deprecated, fgets must be encouraged : it protects you from overflows. You should code this way in C-Style :

char buffer[10];
printf("Your sentence : \n");
fgets(buffer, sizeof buffer, stdin);
printf("Your sentence : %s\n", buffer);

Ouputs :

Your sentence :
1 34 6789012345
Your sentence : 1 34 6789

Nonetheless, you should consider using std::cin with std::string to make correct C++ inputs :

std::string sentence;
std::cout << "Enter a sentence (with spaces): ";
std::getline(std::cin, sentence);
std::cout << "Your sentence (with spaces): " << sentence << std::endl;

Outputs :

Enter a sentence (with spaces): My super sentence
Your sentence (with spaces): My super sentence

Answer 3

A string literal like "Dummy content." is logically const, since any operation that attempts to change its contents results in undefined behaviour.

The definition/initialisation

char *Buffer = "Dummy content.";

however, makes Buffer a non-const pointer to (the first character of) a string literal. That involves a conversion (from array of const char to a char *). That conversion exists in C for historical reasons so is still in C++. However, subsequently using Buffer to modify the string literal - which is what gets(Buffer) does unless the user enters no data - still gives undefined behaviour.

Your "stopped working" error is one manifestation of undefined behaviour.

Giving undefined behaviour is the reason the conversion is deprecated.

Note: gets() is more than deprecated. It has been removed from the C standard, from where it originated, completely because it is so dangerous (no way to prevent it overwriting arbitrary memory). In C++, use getline() instead. It is often not a good idea to mix C I/O function and C++ stream functions on the same input or output device (or file) anyway.

Answer 4

char *Buffer = "Dummy content.";

You should use pointer on const char here because "Dummy content." is not a buffer but pointer on string literal that has type "array of n const char" and static storage duration, so cannot be changed through pointer. Correct variant is:

char const* Literal = "Dummy content.";

But you cannot use it as parameter for gets

gets(Buffer);

It is bad idea and should cause write access exception or memory corruption on writing. You should pass to gets a pointer to a block of memory where received string will be stored. This block should have enough length to store whole string, so in general gets is unsafe, check https://stackoverflow.com/a/4309845/2139056 for more info. But as temporary test solution you can use buffer on stack:

char Buffer[256];
gets(Buffer);

or dynamic allocated buffer:

char* Buffer= new char[256];
gets(Buffer);
//and do not forget to free memory after your output operations
delete [] Buffer;