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Does the C++ Standard Library define this function, or do I have to resort to Boost?

I searched the web and couldn't find anything except Boost, but I thought I'd better ask here.

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  • 6
    You can "resort" to stringstreams too :) Commented Nov 9, 2011 at 13:07

5 Answers 5

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Only partially.

C++11 <string> has std::to_string for the built-in types:

[n3290: 21.5/7]:

string to_string(int val); string to_string(unsigned val); string to_string(long val); string to_string(unsigned long val); string to_string(long long val); string to_string(unsigned long long val); string to_string(float val); string to_string(double val); string to_string(long double val);

Returns: Each function returns a string object holding the character representation of the value of its argument that would be generated by calling sprintf(buf, fmt, val) with a format specifier of "%d", "%u", "%ld", "%lu", "%lld", "%llu", "%f", "%f", or "%Lf", respectively, where buf designates an internal character buffer of sufficient size.

There are also the following that go the other way around:

[n3290: 21.5/1, 21.5/4]:

int stoi(const string& str, size_t *idx = 0, int base = 10); long stol(const string& str, size_t *idx = 0, int base = 10); unsigned long stoul(const string& str, size_t *idx = 0, int base = 10); long long stoll(const string& str, size_t *idx = 0, int base = 10); unsigned long long stoull(const string& str, size_t *idx = 0, int base = 10); float stof(const string& str, size_t *idx = 0); double stod(const string& str, size_t *idx = 0); long double stold(const string& str, size_t *idx = 0);

However, there's nothing generic that you can use (at least not until TR2, maybe!), and nothing at all in C++03.

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9 Comments

Hum, actually there is stoi and friends :)
@mcheema: stroustrup.com/C++11FAQ.html#delegating-ctor then. Either he means Boost, or he's using it as an arbitrary abstraction for the purpose of making a point, or he's wrong.
@mcheema: We can all be interested in spreading good coding style practices, but that doesn't make them any less subjective!
@mcheema Remembering Stroustrup's addiction of using bold italic proportional font for code, I don't think one can ever seriously talk about the style of his code. *joke*
@LightnessRacesinOrbit I haven't seen any more rumblings about this making it into a Technical Specification. Did it die T.T
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No it isn't, even in C++11, but it's proposed for inclusion in Technical Report 2, the next set of std library extensions.

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What's the status of that for C++17, by the way?
According to Wikipedia there will be not TR2 because ISO procedures were changed, but Technical Specifications only (en.wikipedia.org/wiki/…).
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There's no std::lexical_cast, but you can always do something similar with stringstreams:

template <typename T> T lexical_cast(const std::string& str) { T var; std::istringstream iss; iss.str(str); iss >> var; // deal with any error bits that may have been set on the stream return var; }
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thanks but that would be even slower than boost::lexical_cast
A stream based solution isn't going to be as fast as something like snprintf, but you didn't mention performance concerns in your question.
Performance is always a concern, but it is never the only concern. Simplicity, clarity, correctness, and maintainability are also very important. I wouldn't use the above code in a tight loop, but it might be just fine for other situations. A common question in regards to performance on SO is if you've profiled your code, and found that the sacrifice of other concerns are worthwhile :)
If you end up using lexical_cast<string>(string) this will only return the first word, not what you pass in. (You might use it in a templated function or something, not directly.) Something to look out for.
This version can be sped up a lot by making iss static
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No it's a pure Boost thing only.

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6

If you don't want boost then a lightweight library called fmt implements the following:

// Works with all the C++11 features and AFAIK faster then boost or standard c++11 std::string string_num = fmt::format_int(123456789).str(); // or .c_str()

More examples from the official page.

Accessing arguments by position:

format("{0}, {1}, {2}", 'a', 'b', 'c'); // Result: "a, b, c" format("{}, {}, {}", 'a', 'b', 'c'); // Result: "a, b, c" format("{2}, {1}, {0}", 'a', 'b', 'c'); // Result: "c, b, a" format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated // Result: "abracadabra"

Aligning the text and specifying a width:

format("{:<30}", "left aligned"); // Result: "left aligned " format("{:>30}", "right aligned"); // Result: " right aligned" format("{:^30}", "centered"); // Result: " centered " format("{:*^30}", "centered"); // use '*' as a fill char // Result: "***********centered***********"

Replacing %+f, %-f, and % f and specifying a sign:

format("{:+f}; {:+f}", 3.14, -3.14); // show it always // Result: "+3.140000; -3.140000" format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers // Result: " 3.140000; -3.140000" format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}' // Result: "3.140000; -3.140000"

Replacing %x and %o and converting the value to different bases:

format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42); // Result: "int: 42; hex: 2a; oct: 52; bin: 101010" // with 0x or 0 or 0b as prefix: format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42); // Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010"
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I really like the fmt library and like to see its usage promoted. fmt, however, will only do the to string conversion. (Now it does that better than any other C++ library I have seen.) fmt does not do the from string conversion performed by lexical_cast or std::stoxyz functions.

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