Having mostly used Qt and its classes like QString, I wrote a little exercise with plain C++11. I'm looking for two kinds of input: generally improving the code and/or doing it better in an entirely different way (still standard C++11 or C++14, no Boost or anything).The code is for printing doubles with given number of significant digits, by first producing a string using scientific notation, and then manipulating that string to move the dot and pad it with zeros.#include <sstream>#include <iostream>#include <iomanip>#include <vector>#include <algorithm>#include <stdexcept>template <typename T>std::string number_to_sci_string(T number, int precision) {    std::stringstream ss;    ss << std::scientific << std::setprecision(precision-1) << number;    return ss.str();}std::string sci_string_to_normal(const std::string &s) {    auto dot_pos = s.find('.');    auto e_pos = s.find('e');    if (dot_pos == std::string::npos || // dot required        dot_pos < 1 ||  // part1 below must not be empty        e_pos == std::string::npos || // e required        e_pos >= s.length()-1 || // must not be last        e_pos - dot_pos < 2) { // part2 below must not be empty        throw std::invalid_argument(s);    }    auto exponent = std::stoi(s.substr(e_pos + 1), nullptr, 10);    if (exponent == 0) {        return s.substr(0, e_pos);    }    else {        auto part1 = s.substr(0, dot_pos);        auto part2 = s.substr(dot_pos + 1, e_pos - dot_pos - 1);        auto part2length = static_cast<ssize_t>(part2.length());        if (exponent >= part2length) {            auto fill = std::string(exponent - part2length, '0');            return part1 + part2 + fill;        }        else if (exponent < 0) {            auto fill = 0. + std::string(-1 - exponent, '0');            // this is the only case where sign matters            auto sign = (s[0] == '-' || s[0] == '+')                        ? std::string(1, s[0])                        : std::string();            return sign + fill + part1.substr(sign.length()) + part2;        }        else { // 0 < exponent < part2.length()            return part1 + part2.substr(0,  exponent) + . + part2.substr(exponent);        }    }}using namespace std;int main(void){    vector<double> nums{ -2000.0005, -1.2345234, -0.000011345, 0, 0.0000001, 0.1342134, 1.1234, 10000 };    for_each(nums.begin(), nums.end(), [](double &n) {        string sci = number_to_sci_string(n, 5);        string nor = sci_string_to_normal(sci);        cout << n << :  << sci <<  ==  << nor << endl;    });}
Printing doubles using string manipulation
c++;strings;c++11;floating point
number_to_sci_string looks ok, though you could considerably improve performance by chucking the template, restricting to float, double and long double and using snprintf:std::string number_to_sci_string(long double number, int precision) {    std::string s(std::snprintf(0, 0, %.*Le, precision-1, number), '\0');    std::snprintf(&s[0], s.size()+1, %.*Le, precision-1, number);    return s;}Also, I really wonder why you are using a different notion of precision than the standard library...I suggest you be a bit more flexible in what you accept:The exponent e00 should be optional.The fractional part (and decimal point) .00 should be optional.If there's a fractional part, the integer part should be optional.Don't use using namespace std;, even in the implementation-file: You don't control what symbols it contains, and there are no guarantees none are added.The opening brace of main()'s body is, in contrast to all other functions, on a separate line. Why?Prefer the for-range-loop to std::for_each+lambda. The compiler should compile both to the same result, but the former is simpler and looks better.