basic_string::reserve
Sets the capacity of the string to a number at least as great as a specified number.
void reserve(
size_type _Count = 0
);
Parameters
- _Count
The number of characters for which memory is being reserved.
Remarks
Having sufficient capacity is important because reallocations is a time-consuming process and invalidates all references, pointers, and iterators that refer to characters in a string.
The concept of capacity for objects of type strings is the same as for objects of type vector. Unlike vector, the member function reserve may be called to shrink the capacity of an object. The request is nonbinding and may or may not happen. As the default value for the parameter is zero, a call of reserve is a non-binding request to shrink the capacity of the string to fit the number of characters currently in the string. The capacity is never reduced below the current number of characters.
Calling reserve is the only possible way to shrink the capacity of a string. However, as noted above, this request is nonbinding and may not happen.
Example
// basic_string_reserve.cpp
// compile with: /EHsc
#include <string>
#include <iostream>
int main( )
{
using namespace std;
string str1 ("Hello world");
cout << "The original string str1 is: " << str1 << endl;
basic_string <char>::size_type sizeStr1, sizerStr1;
sizeStr1 = str1.size ( );
basic_string <char>::size_type capStr1, caprStr1;
capStr1 = str1.capacity ( );
// Compare size & capacity of the original string
cout << "The current size of original string str1 is: "
<< sizeStr1 << "." << endl;
cout << "The capacity of original string str1 is: "
<< capStr1 << "." << endl << endl;
// Compare size & capacity of the string
// with added capacity
str1.reserve ( 40 );
sizerStr1 = str1.size ( );
caprStr1 = str1.capacity ( );
cout << "The string str1with augmented capacity is: "
<< str1 << endl;
cout << "The current size of string str1 is: "
<< sizerStr1 << "." << endl;
cout << "The new capacity of string str1 is: "
<< caprStr1 << "." << endl << endl;
// Compare size & capacity of the string
// with downsized capacity
str1.reserve ( );
basic_string <char>::size_type sizedStr1;
basic_string <char>::size_type capdStr1;
sizedStr1 = str1.size ( );
capdStr1 = str1.capacity ( );
cout << "The string str1 with downsized capacity is: "
<< str1 << endl;
cout << "The current size of string str1 is: "
<< sizedStr1 << "." << endl;
cout << "The reduced capacity of string str1 is: "
<< capdStr1 << "." << endl << endl;
}
The original string str1 is: Hello world
The current size of original string str1 is: 11.
The capacity of original string str1 is: 15.
The string str1with augmented capacity is: Hello world
The current size of string str1 is: 11.
The new capacity of string str1 is: 47.
The string str1 with downsized capacity is: Hello world
The current size of string str1 is: 11.
The reduced capacity of string str1 is: 47.
Requirements
Header: <string>
Namespace: std