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Showing content from https://cplusplus.com/reference/string/stod/ below:

function template

std::stod

double stod (const string&  str, size_t* idx = 0);double stod (const wstring& str, size_t* idx = 0);

Convert string to double

Parses str interpreting its content as a floating-point number, which is returned as a value of type double.

If idx is not a null pointer, the function also sets the value of idx to the position of the first character in str after the number.

The function uses strtod (or wcstod) to perform the conversion (see strtod for more details on the process). Note that the format accepted by these functions depends on the current locale.

Parameters

str: String object with the representation of a floating-point number.
idx: Pointer to an object of type size_t, whose value is set by the function to position of the next character in str after the numerical value.
This parameter can also be a null pointer, in which case it is not used.

Return Value On success, the function returns the converted floating-point number as a value of type double.

Example

// stod example
#include <iostream>   // std::cout
#include <string>     // std::string, std::stod

int main ()
{
  std::string orbits ("365.24 29.53");
  std::string::size_type sz;     // alias of size_t

  double earth = std::stod (orbits,&sz);
  double moon = std::stod (orbits.substr(sz));
  std::cout << "The moon completes " << (earth/moon) << " orbits per Earth year.\n";
  return 0;
}

Possible output:

The moon completes 12.3684 orbits per Earth year.

Complexity Unspecified, but generally linear in the number of characters interpreted.

Data races Modifies the value pointed by idx (if not zero).

Exceptions If no conversion could be performed, an invalid_argument exception is thrown.
If the value read is out of the range of representable values by a double (in some library implementations, this includes underflows), an out_of_range exception is thrown.

An invalid idx causes undefined behavior.