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C++20 STL Cookbook

Join Container Elements into a String

Explore how to join elements from any container into a formatted string using iterators and ostream in C++. Understand how to avoid trailing separators and create flexible join functions that work with various container types and views.

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Sometimes, there is no algorithm in the library to accomplish a task at hand. We can use iterators, with the same techniques as the algorithms library, to easily write one.

For example, we often need to join elements from a container, with separators, into a string. One common solution is to use a simple for() loop:

for(auto v : c) cout << v << ', ';

The problem with this otherwise simple solution is that it leaves a trailing separator Trailing separator refers to a separator character or symbol placed at the end of a series or list, except for the last item, to visually indicate the separation between elements. :

vector<string> greek{ "alpha", "beta", "gamma",
    "delta", "epsilon" };
for(auto v : greek) cout << v << ", ";
cout << '\n';

Output:

alpha, beta, gamma, delta, epsilon,

This may be fine in a testing environment, but in any production system, that trailing comma is unacceptable.

The ranges::views library has a join() function, but it doesn't provide a separator:

auto greek_view = views::join(greek);
...