parsing a formatted string

[JUANDENT]

using the format library in C++20, and having done this:

int val = 25;
string str = std::format( "{0:2}", val);

How can we get the val from str, something like this:

int v = std::parse(str);
// or:
int v = std::parse(str, "{0:2}");

where parse does not exist as far as I know


Regards,
Juan

[Peter87]

There isn't anything like std::format for parsing text input but there is a proposal:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1729r1.html

In the meantime I recommend using std::istringstream.

[JUANDENT]

I have this:

std::istringstream is{"5,600"};
is.imbue(std::locale{"en_US"});
double dou;
is >> dou;

it works but what if it had a currency symbol?

std::istringstream is{"$5,600"};
is.imbue(std::locale{"en_US"});
double dou;
is >> dou;

it does not work! dou no longer gets its value to 5600.00000 instead it has value 0.0000000

[Peter87]

Pass the string as argument to the constructor and read from it the same way as you would from std::cin.

#include <iostream>
#include <sstream>

int main ()
{
	std::istringstream iss("hello 123");
	
	std::string word;
	iss >> word;
	
	int num;
	iss >> num;
	
	std::cout << num << " " << word << "\n"; // prints "123 hello"
}

[JLBorges]

> what if it had a currency symbol?

std::get_money https://en.cppreference.com/w/cpp/io/manip/get_money

Example: https://en.cppreference.com/w/cpp/io/manip/get_money#Example

[seeplus]

There is sscanf() which works with null-terminated strings. See (3) of:
https://en.cppreference.com/w/c/io/fscanf

Consider:

#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <format>
#include <string>
#include <cstdio>

int main() {
	const int val { 25 };
	const auto str { std::format("{0:2}", val) };

	std::cout << str << '\n';

	int val1 {};

	std::sscanf(str.c_str(), "%i", &val1);

	std::cout << val1 << '\n';
}

[seeplus]

To just obtain a number from std::string, there are the stoxxx functions. See https://cplusplus.com/reference/string/

However these (and from_chars() ) just convert one value. They're not really the opposite of std::format() which can deal with multiple values. Until std:parse() comes, the nearest is sscanf().