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In C++ standard library there are functions to convert from string to numeric types:

stoi stol stoll stoul stoull stof stod stold

but I find it tedious to use them in template code. Why there are no template functions something like:

template<typename T> T sto(...)

to convert strings to numeric types?

I don't see any technical reason to not have them, but maybe I'm missing something. They can be specialised to call the underlying named functions and use enable_if/concepts to disable non numeric types.

Are there any template friendly alternatives in the standard library to convert string to numeric types and the other way around in an efficient way?

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  • Does this answer your question? Why is the `std::sto`... series not a template? Commented Feb 20, 2020 at 16:31
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    @Boiethios not really - the answers from that question explain the rationale behind the "why", but they don't come with practical solutions like the accepted answer. I've edited my question to ask for alternative to better state what I need Commented Feb 24, 2020 at 6:43

5 Answers 5

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44

Why there are no template functions something like:

C++17 has such generic string to number function, but named differently. They went with std::from_chars, which is overloaded for all numeric types.

As you can see, the first overload is taking any integer type as an output parameter and will assign the value to it if possible.

It can be used like this:

template<typename Numeric> void stuff(std::string_view s) { auto value = Numeric{}; auto [ptr, error] = std::from_chars(s.data(), s.data() + s.size(), value); if (error != std::errc{}) { // error with the conversion } else { // conversion successful, do stuff with value } }

As you can see, it can work in generic context.

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4 Comments

C++ has destructuring now? :o Structured binding declaration
Of course! It even work with simple structs or if given the right interface, classes too.
It is worth noting that this won't work with floating point precision, as mentioned on Reddit and as I just tested by myself.
@PatrizioBertoni as far as I know it's supposed to work with a C++17 complete standard library. MSVC implement the floating point version of the functions.
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It's not a template, and it doesn't work with locales but if that isn't a show stopper then C++17 already has what you want: std::from_chars

There are overloads for all of the integer and floating-point types and the interface is the same except for the last parameters which are different for the integer and floating-point types respectively (but if the default is fine then you don't need to change anything). Because this is not a locale-aware function it is also quite fast. It will beat any of the other string to value conversion function and generally it is by orders of magnitude.

There is a very good CPPCON video about <charconv> (the header from_chars lives in) by Stephan T. Lavavej that you can watch about its usage and performance here: https://www.youtube.com/watch?v=4P_kbF0EbZM

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1 Comment

@NathanOliver: stoi and its friends (the conversions mentioned in the question) also don't work with locales, so that isn't a showstopper.
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You wouldn't gain much because in an expression like

int x = sto("1");

There is no (easy) way to deduce the desired type for the template parameter. You would have to write

int x = sto<int>("1");

which to some extend defeats the purpose of providing a generic function. On the other hand, a 

template<typename T> void sto(std::string x,T& t);

would be of good use as you realized. In C++17 there is std::from_chars, which does more or less exactly that (it is no template but a set of overloads and it takes pointers to chars instead of a string, but thats only minor details).

PS There is no easy way to deduce the desired type in the above expression, but there is a way. I dont think the core of your question was exactly the signature you asked for, and I dont think the following is a good way to implement it, but I knew that there is a way to make the above int x = sto("1"); compile and I was curious to see it in action.

#include <iostream> #include <string> struct converter { const std::string& x; template <typename T> operator T() { return 0;} }; template <> converter::operator int() { return stoi(x); } template <> converter::operator double() { return stod(x); } converter sto(const std::string& x) { return {x}; } int main() { std::string s{"1.23"}; int x = sto(s); double y = sto(s); std::cout << x << " " << y; }

This works as intended, but it has severe downsides, maybe most importantly it allows to write auto x = sto(s);, ie it is easy to use wrong.

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6 Comments

I think relying on the implicit conversion here is a good idea. Trying to disable auto is a problem though. Typically, I have seen that done by putting a private const reference in a class which is only initialized by valid methods. I can't see how one would leverage that here though because we kind of have to construct a whole converter object before proceeding. Hmmm....
I can see value despite the non-deduced type parameter - as the question says, the motivation is to be able to use from within template code, where you're converting to a type that varies between instantiations.
What's the fundamental problem with auto x = sto(s) ? This particular implementation breaks because converter::x is a reference which goes out of scope, but that's fixable. Just remove the reference and rely on std::string's move semantics.
@MSalters yes, it was the reference that I thought is problematic, but you are right, no need to use a reference. What actually disturbs me more is that it appears to be a function but the actual functionality is in converter, also I am not sure if using a template conversion operator was the best choice, things that could be fixed. Maybe it isnt that bad as I initially thought
I don't think there is any problem with the const reference here. My understanding is that the const reference will preserve the lifetime of the string until the converter is destructed (herbsutter.com/2008/01/01/…)
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Solution compatible with all (even older C++ compilers like C++-98 ones) is to use boost::lexical_cast which is a template to convert between numeric and string types in both ways.

Example:

short myInt = boost::lexical_cast<short>(*argv); std::string backToString = boost::lexical_cast<std::string>(myInt);

See: https://www.boost.org/doc/libs/1_42_0/libs/conversion/lexical_cast.htm

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4

On older C++ versions, stringstream is your friend. If I understand correctly, then the following might be interesting to you. It is C++11.

https://wandbox.org/permlink/nUNiUwWWTr7a0NXM

#include <sstream> #include <string> #include <iostream> template<typename T, typename String> T sto(const String & str) { T val; std::stringstream ss(str); ss >> val; return val; } template<typename T, typename String> void sto(const String & str, T & val) { std::stringstream ss(str); ss >> val; } int main() { std::cout << sto<float>("1.1") << ", " << sto<int>(std::string{"2"}); // An alternative version that infers the type double d; sto("3.3", d); std::cout << ", " << d; }

This method works in C++11 and is pretty general. In my experience, this method is robust, but not the most performant.

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2 Comments

Yes, this is what I've used, but the performance is below named functions which sometimes is not desired
One drawback of this method is that it cannot throw an error if the input string is not convertible to the type T

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