Gaining access to the tuple by string template param

Question

A standard tuple in C++ 11 allows access by the integer template param like this:

tuple<int, double> test;
test.get<1>();

but if I want get access by the string template param:

test.get<"first">()

how can I implement it?

First, it is `std::get<1>(test)` (ok, that may change in C++1z). Second, you cannot take pointer like this for template parameter. And finally, why do you want to use c-string literal instead of index ? — Jarod42, Jan 14 '16 at 00:25
If you want a runtime string it is not possible. If you want a static string there are better options by using type_tags like in `boost::fusion::map`. — alfC, Jan 14 '16 at 00:35
In the next step I want to implement relationships class for database coprocessor. Column called string names in database tables. — COUNTERKILL, Jan 14 '16 at 00:42
@COUNTERKILL So why not use `std::map` instead of `std::tuple` ? That will alow you to write `test["first"]`. — mikedu95, Jan 14 '16 at 00:55
No. A Columns in databases can have a different types. If I will be use std::map, I can't get the same access to all columns. — COUNTERKILL, Jan 14 '16 at 01:14
Do you know the types at compile type? If not then you can can use `std::map` or `std::map`. (take a look at `boost::property_tree`). — alfC, Jan 14 '16 at 02:11

score 1 · Answer 1 · answered Jan 14 '16 at 09:27

You can create custom constexpr cast function. I just wanted to show that what the OP wants is (almost) possible.

#include <tuple>
#include <cstring>

constexpr size_t my_cast(const char * text)
{
    return !std::strcmp(text, "first") ? 1 :
           !std::strcmp(text, "second") ? 2 :
           !std::strcmp(text, "third") ? 3 :
           !std::strcmp(text, "fourth") ? 4 :
           5;
}

int main()
{
    std::tuple<int, double> test;
    std::get<my_cast("first")>(test);
    return 0;
}

This can be compiled with C++11 (C++14) in GCC 4.9.2. Doesn't compile in Visual Studio 2015.

R Sahu · Accepted Answer · 2016-01-14T00:32:50.183

0

First of all, std::tuple::get is not a member function. There is a non-member function std::get.

Given,

std::tuple<int, double> test;

You cannot get the first element by using:

std::get<"first">(test);

You can use other mnemonics:

const int First = 0;
const int Second = 1;
std::get<First>(test);
std::get<Second>(test);

if that makes the code more readable for you.

edited Jan 14 '16 at 00:32

answered Jan 14 '16 at 00:25

R Sahu

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score -1 · Answer 3 · answered Jan 14 '16 at 01:32

R Sahu gives a couple of good mnemonics, I wanted to add another though. You can use a C style enum (i.e. non-class enum):

enum TupleColumns { FIRST, SECOND };

std::get<FIRST>(test);

If you combine enums with a smart enum reflection library like so: https://github.com/krabicezpapundeklu/smart_enum, then you can create a set of enums that have automatic conversions to and from string. So you could automatically convert column names into enums and access your tuple that way.

All this requires you to commit to your column names and orders at compile time. In addition, you'll always need to use string literals or constexpr functions, so that you can get the enum value as constexpr to use it this way.

constexpr TupleColumns f(const char *);
constexpr auto e = f("first");
std::get<e>(test);

I should probably add a warning at this point: this is all a fairly deep rabbit hole, fairly strong C++ is required. I probably would look for a different solution in the bigger picture, but I don't know your bigger picture well enough, nor do I know the level of your C++ nor your colleagues (assuming you have them).

Gaining access to the tuple by string template param

3 Answers3