Tuesday, May 8, 2012

stl vector as array

Using Vectors as Ordinary Arrays

The C++ standard library does not state clearly whether the elements of a vector are required to be in contiguous memory. However, it is the intention that this is guaranteed and it will be fixed due to a defect report. Thus, you can expect that for any valid index i in vector v, the following yields true:

&v[i] == &v[0] + i

This guarantee has some important consequences. It simply means that you can use a vector in all cases in which you could use a dynamic array. For example, you can use a vector to hold data of ordinary C-strings of type char* or const char*:

std::vector<char> v; // create vector as dynamic array of chars
v.resize(41); // make room for 41 characters (including
strcpy(&v[0], "hello, world"); // copy a C-string into the vector
printf("%s\n", &v[0]); // print contents of the vector as C-string

Of course, you have to be careful when you use a vector in this way (like you always have to be careful when using dynamic arrays). For example, you have to ensure that the size of the vector is big enough to copy some data into it and that you have an '\0' element at the end if you use the contents as a C-string. However, this example shows that whenever you need an array of type T for any reason (such as for an existing C library) you can use a vector<T> and pass the address of the first element.

Note that you must not pass an iterator as the address of the first element. Iterators of vectors have an implementation-specific type, which may be totally different from an ordinary pointer:

printf("%s\n", v.begin()); // ERROR (might work, but not portable)
printf("%s\n", &v[0]); // OK

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