String to floating-point implementation

While working on an implementation of a scanf-like method for an input stream class, I realized that I needed to also implement a string to floating-point function to be used by this method for converting the read floating-point input string. Like my stoi template function, I created this stof template function that can convert a string to a float, double, or long double. While this function does detect overflows and underflows, it doesn’t do so perfectly and may sometimes miss some of those errors, resulting in a value of INF or NaN.

#include <cctype>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <stdexcept>
#include <cfloat>
#include <iostream>
using namespace std;

/* float, double, or long double */
template <typename type>
type stof(const char* s) {
	int i;
	type \
		ret = 0,			//Final return value
		num = 0,			//accumulator
		place = 1;		//digit place 10's value
	bool neg = false;	//Negative or positive number			
	const char *t;
	bool pineapple = false;		//Tells if we are calculating the whole or fractional part
	while (isspace(*s)) s++;	//Skip whitespace
	if (*s == '-') { neg = true; s++; }		//Check for negative
	else if (*s == '+') s++;
	if ((t = strchr(s, '.')) == NULL)		//Find first digit of the whole number part
		if ((t = strchr(s, 'e')) == NULL)
			t = s + strlen(s);
	for (t--; t >= s; t--, place *= 10) {	//Calculate the whole number part
		goto stof_smoothie;
		stof_forloop_1: ;
	//Skip if no decimal point
	if ((t = strchr(s, '.')) == NULL) goto stof_blend_ice;
	//Calculate the fractional part
	pineapple = true;
	//Loop through each number after decimal point.
	//I didn't use isdigit in the for loop compare because
	//later on i have to check if it's a valid digit
	//and poop out an error if it's not.
	for (t++, place = 0.1;
		*t != 'e' && *t != 0;
	t++, place /= 10) {
		goto stof_smoothie;
		stof_forloop_2: ;
	if ((t = strchr(s, 'e')) == NULL)
		return ret;
	//Multiply by 10^x if suffixed with 'e'
	else {
	//Where x comes right after e
		i = atoi(t + 1);
	//Divide by 10s if negative
		if (i < 0) {
			for (; i < 0; i++) {
				num = ret / 10;
	//Dividing should make a smaller number, if not...there was an underflow
				if (num >= ret) throw underflow_error("underflow");
				else ret = num;
		} else {
	//Multiply by 10s if positive
			for (; i > 0; i--) {
				num = ret * 10;
	//Multiplying should make a bigger number, if not...there was an overflow
				if (num <= ret) throw overflow_error("overflow 0");
				else ret = num;
	return ret;

//Performed in both loops where the whole and fractional
//parts are calculated. I know goto is considered bad
//but this time it's just genius. I save more space this

//Not sure if this applies to floating numbers
//but place value would loop back to 0 if there
//was an over/underflow
	if (!place)
		if (neg) throw underflow_error("underflow");
		else throw overflow_error("overflow 1");

//Validate and convert digit
	if (!isdigit(*t)) throw invalid_argument("Invalid string");
	num = (*t - '0') * place;
//Basically, it's an overflow or underflow if the result is not logical
	if (neg) {
		if ((type)(ret - (num ? num : place) > ret))
			throw underflow_error("underflow");
		ret -= num;
		if (ret >= 0 && num != 0) throw underflow_error("underflow");
	} else {
		if ((type)(ret + (num ? num : place) <= ret)) throw overflow_error("overflow 2");
			ret += num;
		if (ret <= 0 && num != 0) throw overflow_error("overflow 3");
	if (pineapple) goto stof_forloop_2;
	else goto stof_forloop_1;

int main() {
	cout << stof<float>("3.14e-8") << endl;

5 thoughts on “String to floating-point implementation

    • Interesting, looks likes you put a lot of work into your algorithm. I tested it on a little-endian machine and it comes out with the same results as my function. FLT_MIN and FLT_MAX also come out with same accurate values for both functions. So, maybe you can point out which string instance would make my function return an inaccurate value, I’m unable to find one.

  1. Sure. I’ve used double precision. It is guarranted to provide 16 valid significant digits. Then I’ve added “cout.precision(16)” in main(), copypasted my code into your one to have both functions available, renamed my atof() to atof2(), so there is no conflict with standard library.
    Here is my main():

    cout << stof(“3.141592653589793e+307”) << endl;
    cout << atof2("3.141592653589793e+307") << endl;

    The number used has 16 significant digits. On my machine with GCC compiler I get following results:


    Then I went further and decided to check exact hex values of returned double numbers. After some typecasting like:

    unsigned long long int *z;
    double p;

    p = stof(“3.141592653589793e+307”);
    z = (unsigned long long int*)&p;
    cout << hex << *z << endl;

    I've got that our functions return following values:

    your stof(): 0x7fc65e6f105a3049
    my atof2(): 7fc65e6f105a304c

    What is more interesting I’ve tried standard library atof(), which returns 0x7fc65e6f105a3050. Then I’ve decided to calculate the real value of all three results using ttmath online calculator with 512 bit mantissa. Dissecting doubles we get:

    Sign bit in all three results is of course cleared. Exponent in all three results is 2044, 1021 after subtracting offset. 52-bit significands are:

    1792680968990793 – for your stof()
    1792680968990796 – for my atof2()
    1792680968990800 – for stdlib atof()

    Then I apply the formula “x = 2^exp * (1 + significand * 2^-52)” to get the real values, using with 512-bit precision. I’ve placed a vertical bar after 16 significant digits:

    3.141592653589791|3220 e+307 – for your stof(), error is -1.678e+292
    3.141592653589792|8188 e+307 – for my atof2(), error is -0.181e+292
    3.141592653589794|8147 e+307 – for stdlib atof(), error is +1.814e+292

    Then, my code provides the most accurate conversion possible (changing the significand by +1 or -1 increases the modulo of error). To my surprise, my code beats even standard library function (and in fact your code beats it too, by small factor). It may be however my standard library is not so modern (I’ve used GCC 4.4.5 compiler).

    • ADDED: It turned out that standard library atof() errors only happened for some obscure stdlib version I’ve used. After I’ve changed the standard library, atof() results are identical to my code results, at least for a few values I’ve checked. Then my claim that “my code beats even the standard library” is false in general.

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