Why is there no std::stou?

109
votes

C++11 added some new string conversion functions:

http://en.cppreference.com/w/cpp/string/basic_string/stoul

It includes stoi (string to int), stol (string to long), stoll (string to long long), stoul (string to unsigned long), stoull (string to unsigned long long). Notable in its absence is a stou (string to unsigned) function. Is there some reason it is not needed but all of the others are?

related: No "sto{short, unsigned short}" functions in C++11?

3
c++stringc++11std
My question was intended to be more along the lines of "is there some non-obvious drawback of just using stoul". Obviously that will mess with template instantiation, but is there anything else that I'm not considering? Comments on why it was left out would be nice but secondary. – David Stone
@NicolBolas I cannot see why this is not constructive. It is a perfectly valid question as I cannot see any reason for this inconsistency and anwers may give insights into some possibly existing valid but not that obvious reason for it. – Christian Rau
@SethCarnegie Well, what your platform (and maybe the majority of platforms) does is just irrelevant, because an unsigned long just is no unsigned int. – Christian Rau
@SethCarnegie: on my typical computer, unsigned long is 64 bits, and unsigned int 32. They are different types, and can't be assumed to be the same as each other. – Mike Seymour
@NicolBolas Like said, the OP (and me) doesn't know it is speculative, as there could just be a perfect valid reason for it burried deep in the language internals of C++. But since you say it's speculative I guess there is no such reason. But again, maybe a C++11-responsible person can still answer it. This is no "Wah wah, where is that damn stou"-question, but a question asking for a possibly definite reason for this obvious inconsistency. If you know there is no such reason, then well, post it as an answer. – Christian Rau

3 Answers

30
votes

The most pat answer would be that the C library has no corresponding “strtou”, and the C++11 string functions are all just thinly veiled wrappers around the C library functions: The std::sto* functions mirror strto*, and the std::to_string functions use sprintf.

Edit: As KennyTM points out, both stoi and stol use strtol as the underlying conversion function, but it is still mysterious why while there exists stoul that uses strtoul, there is no corresponding stou.

23
votes

I've no idea why stoi exists but not stou, but the only difference between stoul and a hypothetical stou would be a check that the result is in the range of unsigned:

unsigned stou(std::string const & str, size_t * idx = 0, int base = 10) {
    unsigned long result = std::stoul(str, idx, base);
    if (result > std::numeric_limits<unsigned>::max()) {
        throw std::out_of_range("stou");
    }
    return result;
}

(Likewise, stoi is also similar to stol, just with a different range check; but since it already exists, there's no need to worry about exactly how to implement it.)

0
votes
unsigned long ulval = std::stoul(buf);
unsigned long mask = ~0xffffffffl;
unsigned int uival;
if( (ulval & mask) == 0 )
    uival = (unsigned int)ulval;
else {
    ...range error...
}

Using masks to do this with the expected value size in bits expressed in the mask, will make this work for 64-bit longs vs 32-bit ints, but also for 32-bit longs vs 32-bit ints.

In the case of 64-bit longs, ~0xffffffffl will become 0xffffffff00000000 and will thus see if any of the top 32 bits are set. With 32-bit longs, it ~0xffffffffl becomes 0x00000000 and the mask check will always be zero.