Overloading subscript operator causes access violation

Question

I get an access violation error when I use a self written Buffer class:

template<typename T> 
class Buffer {
public:
    Buffer(T *data, size_t len);
    Buffer(size_t len);

    size_t len();

    operator T*();
    T& operator[] (const int x) const {
        return this->data[x];
    };

private:
    T *data;
    size_t _len;
};

int main() {
    Buffer<char> b("123", 3);
    b[0] = 0; // This line causes "Access violation writing location 0x003c8830".

    return 0;
}

Why is that? What am I doing wrong?

Removed [c] tag, this is clearly C++. Next, we won't know how that happened until we see the constructors, but I have a vague idea... — Xeo
– Xeo, Commented May 30, 2012 at 11:06
String literals are read only, guessing the constructor just points to the string passed in. Can you post the constructor definitions? — hmjd
– hmjd, Commented May 30, 2012 at 11:07

Sergey Kalinichenko · Accepted Answer · 2012-05-30 11:09:14Z

1

The code that causes the issue is not in the post, but it's very likely your constructor: if you assign the pointer data to the member data, then you store the string constant inside your class. Then code of the main than tries to write into non-writable memory, causing access violation.

You can fix your constructor to avoid this problem:

Buffer(T *dataIn, size_t len) {
    data = new T[len];
    memcpy(data, dataIn, len*sizeof(T));
}

answered May 30, 2012 at 11:09

Sergey Kalinichenko

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1 Comment

Xeo Over a year ago

Hey, don't go stealing my mystical glass orb of psychic debugging! ;)

Community · Accepted Answer · 2017-05-23 12:20:18Z

1

"123" is a string literal stored in read only memory,
When you do:

b[0] = 0

You are trying to write to the read only memory thus invoking Undefined Behavior.

You need to allocate dynamic memory to data so that it owns the memory and your code then modify's the owned memory and not the non writable memory which it does not own.
Also, Do make sure You follow the Rule of Three.

edited May 23, 2017 at 12:20

CommunityBot

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answered May 30, 2012 at 11:10

Alok Save

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1 Comment

Luchian Grigore Over a year ago

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std''OrgnlDave · Accepted Answer · 2012-05-30 11:10:55Z

1

String literals such as "123" are non-writeable. You attempt to assign a value to it when you do b[0] = 0;

In addition, string literals have a terminating NULL character, making that 4 long.

You can specialize you constructor to copy const char inputs.

answered May 30, 2012 at 11:10

std''OrgnlDave

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Comments

chrisaycock · Accepted Answer · 2012-05-30 11:11:32Z

1

You don't show the definition of the constructor, but I bet you're using the assignment operator to copy the C string to Buffer::data. You instead need to use strcpy(). Also, you'll need to allocate 3 + 1 bytes for Buffer::data to ensure that you have enough room for the nul terminator.

answered May 30, 2012 at 11:11

chrisaycock

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