2

C++17 brought us a nicer way to iterate through map using the Structured binding, as shown in this example .

I am bound to use C++14 but have a lot of repeating code that iterates over maps.

In C++14 it looks like this:

for (auto const& item : myMap)
{
    std::string key = x.first;
    int value = x.second;
    //Code...
}

Is there a way to create either a template / define / using / anything else that will allow me the convenience of iterating in this manner but still compile as a C++14 application?

for( auto const& [key, value] : myMap)
{
    //Code...
}
Nicol Bolas
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joepol
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2 Answers2

2

You cannot initialize variables inside the range-based loop until C++20, so the best option, that I can see to use such macro:

#define STRUCTURED_BINDING(a, b, map_item) \
         const auto &a = map_item.first; \
         const auto &b = map_item.second;

So then in your code, you can write something like that:

for(const auto& item: a)
{
    STRUCTURED_BINDING(fst, snd, item);
    std::cout << fst << " " << snd << std::endl;
}

You can improve this macro to be used with tuple (get<0>, get<1>, ...), rewriting macro to use variadic arguments.

Midren
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  • Or you could use `std::tie` – janekb04 Aug 16 '20 at 08:37
  • Nice... But what if the key and values should be non-const in the scope of for loop. Or may be also just copy for primitive types? Anyways+1 for the macro... Seems to me extendable for other cases! – MyClass Aug 16 '20 at 09:05
0

Maybe you can hack something here - but you'd better not.

While @Midren's suggestion will indeed work, I want to suggest that you not do this.

It's true that the C++14 loop is more verbose than in C++17. But - it is easy to understand and well-recognized. Introducing a macro, or a custom gadget - may save you a few characters, but will cost you in Readability and Maintainability. If your macro-based approach were to become very popular, then the readability would become a non-issue and the maintainability would be almost trivial (just making sure everyone uses the same gadget and not redefining it, in larger projects). But - that will never happen, since we already have C++17.

Instead, I would tweak the naming and perhaps spacing to improve clarity. For example, suppose it's a map of foo's to bar's:

for (auto const& foo_bar_pair : myMap) {
    const auto& foo = foo_bar_pair.first;
    const auto& bar = foo_bar_pair.second;

    //Code with `foo` and `bar`
}
einpoklum
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