Constructors That Use an Lvalue Reference

The copy constructor looks like this:

basic_string(const basic_string& str);

It initializes a new string object using a string argument:

string mel("I'm ok!");
string ida(mel);

Here, ida would get a copy of the string managed by mel.

The next constructor additionally requires that you specify an allocator:

basic_string(const basic_string& str, const Allocator&);

The following relationships hold after either of these two constructors is called:

• The data() method returns a pointer to an allocated copy of the array whose first element is pointed to by str.data().

• The size() method returns the value of str.size().

• The capacity() method returns a value at least as large as size().

Moving along, the next constructor lets you set several items:

basic_string(const basic_string& str, size_type pos, size_type n = npos,
    const Allocator& a = Allocator());

The second argument pos specifies a location in the source string from which to begin the copying:

string att("Telephone home.");
string et(att, 4);

Position numbers begin with 0, so position 4 is the p character. Thus, et is initialized to "phone home.".

The optional third argument n specifies the maximum number of characters to copy. Thus, this initializes pt to the string "phone":

string att("Telephone home.");
string pt(att, 4, 5);

However, this constructor does not go past the end of the source string; for example, the following stops after copying the period:

string pt(att, 4, 200)

Thus, the constructor actually copies a number of characters equal to the lesser of n and str.size() - pos.

This constructor requires that pos <= str.size()—that is, that the initial position copied to is inside the source string; if this is not the case, the constructor throws an out_of_range exception. Otherwise, if copy_len represents the lesser of n and str.size() - pos, the following relationships hold after the constructor is called:

• The data() method returns a pointer to a copy of copy_len elements copied from the string str, starting with position pos in str.

• The size() method returns copy_len.

• The capacity() method returns a value at least as large as size().