public member function
size_type capacity() const;
size_type capacity() const noexcept;
Return size of allocated storage
Returns the size of the storage space currently allocated for the basic_string, expressed in terms of characters.
This capacity is not necessarily equal to the string length. It can be equal or greater, with the extra space allowing the object to optimize its operations when new characters are added to the basic_string.
Notice that this capacity does not suppose a limit on the length of the basic_string. When this capacity is exhausted and more is needed, it is automatically expanded by the object (reallocating it storage space). The theoretical limit on the length of a basic_string is given by member max_size.
The capacity of a basic_string can be altered any time the object is modified, even if this modification implies a reduction in size or if the capacity has not been exhausted (this is in contrast with the guarantees given to capacity in vector containers).
The capacity of a basic_string can be explicitly altered by calling member reserve.
The size of the storage capacity currently allocated for the basic_string.
Member type size_type is an unsigned integral type.
// comparing size, length, capacity and max_size
int main ()
std::string str ("Test string");
std::cout << "size: " << str.size() << "\n";
std::cout << "length: " << str.length() << "\n";
std::cout << "capacity: " << str.capacity() << "\n";
std::cout << "max_size: " << str.max_size() << "\n";
A possible output for this program could be:
Unspecified, but generally constant.
The object is accessed.
No-throw guarantee: this member function never throws exceptions.