How can I iterate through variables?

Question

If I have the code:

T a;
T b;
T c;
// ...
T z;

How can I iterate through them without creating std::vector<T&> of them?

Any beautiful solution, something like (pseudo):

for (auto& it : [a, b, c, d, e, f]) {
    // ...
}

(Without copies.)

Generally you can't. You can create a temporary vector containing *copies of the values* of the variables though. — Some programmer dude, Jul 22 '16 at 08:03
You might want to reconsider why you are using so many variables here though. What is the purpose of the code? What is the *actual* problem you want so solve? Maybe even rethink your design? — Some programmer dude, Jul 22 '16 at 08:05
@JoachimPileborg I have a struct like `struct address_t { string state, string city, string street, ... }` and now I need to join all its *non-empty* parts to `string fulladdress`. — vladon, Jul 22 '16 at 08:17
Just to add, @juanchopanza and others, you actually **can** create a vector of reference *wrappers*. See http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper — adnan_e, Jul 22 '16 at 09:40

Jarod42 · Accepted Answer · 2016-07-22T08:14:53.360

11

for (auto& var : {std::ref(a), std::ref(b), std::ref(c), std::ref(d), std::ref(e), std::ref(f)}) {
    // ...
}

Should do the job.

edited Jul 22 '16 at 08:14

answered Jul 22 '16 at 08:04

Jarod42

203,559
14
181
302

1

Is there any reason why you wouldn't simply write `for (auto it : {&a, &b, &c}) ...` ? – H. Guijt Jul 22 '16 at 08:09
@H.Guijt: To have a more transparent usage (you have to dereference `it`), but it is a valid alternative. – Jarod42 Jul 22 '16 at 08:46
`var` is then const and can not be modified, correct? why? – ph_0 Jan 21 '20 at 15:40

Some programmer dude · Answer 2 · 2016-07-22T08:54:09.943

2

If you don't want to actually modify the "variables" then you might be able do something like

// TODO: Put your own variables here, in the order you want them
auto variables = { a, b, c, .... };

// Concatenate all strings
std::string result = std::accumulate(std::begin(variables), std::end(variables), "",
    [](std::string const& first, std::string const& second)
    {
        return first + ' ' + second;  // To add spacing
    });

Note that this requires all "variables" to be of the same type (std::string). If you have a variable that is not a string you can use std::to_string to convert them in the first step.

edited Jul 22 '16 at 08:54

answered Jul 22 '16 at 08:23

Some programmer dude

400,186
35
402
621

Your `initializer_list` does a copy. – Jarod42 Jul 22 '16 at 09:05
@Jarod42 And in most cases it's really nothing to worry about. We don't know the use-case of the OP, if the OP is doing this multiple times a second then yes it might be an issue. If the OP is doing once or twice during a ten-hour run-time then it's really insignificant. It's all about context (which we don't have). If this is measured to be a bottleneck then maybe use `std::cref` instead. – Some programmer dude Jul 22 '16 at 09:13
OP stated explicitly, *"(Without copies.)"*. But I mostly agree in practice. – Jarod42 Jul 22 '16 at 09:16

How can I iterate through variables?

2 Answers2