How to read/write a stucture to a file

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C++

file

struct DATA
{
    int num;
    string str;
};

I don't know how to read (maybe also write)this structure from a text file because there is a string!
I wrote like these:

WriteFile(hFile, &data, sizeof(data)…

and read

DATA data;
ReadFile(hFile, &data, sizeof(DATA)…

But, everytime I run the program, I got a _BLOCK_TYPE_IS_VALID(pHead->nBlockUse) error. Maybe this is because I didn't allocated memory for the str, actually I don't know how to do it.
I know it works if I use char str[STRING_LENGTH] but the length of the string isn't constant.

So how should I deal with it? Or is there other way to do the same work?