C++11’s to_string function (from the <string> header) returns the string representation of its numeric argument. The function is overloaded for types int, unsigned int, long, unsigned long, long long, unsigned long long, float, double and long double.

C++11 provides eight functions (Fig. 21.13; from the <string> header) for converting string objects to numeric values. Each function attempts to convert the beginning of its string argument to a numeric value. If no conversion can be performed, each function throws an invalid_argument exception. If the result of the conversion is out of range for the function’s return type, each function throws an out_of_range exception.

Consider an example of converting a string to an integral value. Assuming the string:

string s("100hello");

the following statement converts the beginning of the string to the int value 100 and stores that value in convertedInt:

int convertedInt = stoi(s);

Each function that converts a string to an integral type actually receives three parameters—the last two have default arguments. The parameters are:

• A string containing the characters to convert.

• A pointer to a size_t variable. The function uses this pointer to store the index of the first character that was not converted. The default argument is a null pointer, in which case the function does not store the index.

• An int from 2 to 36 representing the number’s base—the default is base 10.

So, the preceding statement is equivalent to

int convertedInt = stoi(s, nullptr, 10);

Given a size_t variable named index, the statement:

int convertedInt = stoi(s, &index, 2);

converts the binary number "100" (base 2) to an int (100 in binary is the int value 4) and stores in index the location of the string’s letter "h" (the first character that was not converted).

The functions that convert strings to floating-point types each receive two parameters:

• A string containing the characters to convert.

• A pointer to a size_t variable where the function stores the index of the first character that was not converted. The default argument is a null pointer, in which case the function does not store the index.

Consider an example of converting a string to an floating-point value. Assuming the string:

string s("123.45hello");

the following statement converts the beginning of the string to the double value 123.45 and stores that value in convertedDouble:

double convertedDouble = stod(s);

Again, the second argument is a null pointer by default.