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Nested replacement field

As explained in Learn C++ by Example

A nested replacement field is a replacement field embedded within another replacement field to supply a formatting value dynamically.

A nested replacement field places curly braces inside a format placeholder so another value can supply part of the formatting specification dynamically. In `std::format("{: ^{}}", element, 6)`, the inner field supplies the centered field width. section 2.2.5

Centering a number triangle

A triangle display centers each number in a block of six characters. Replacing the fixed width with an inner field makes that width calculable, although the example keeps the value `6` and encourages experimentation. section 2.2.5

std::format("{: ^{}}", element, 6);

Inspect the compact `std::format` expression that uses an inner replacement field for the width. section 2.2.5

How Learn C++ by Example covers it

How deep it goes
Defines the syntax and demonstrates dynamic width, but stops short of using a calculated width in the final example.
Examples
Shows centered numbers in a triangle, using `std::format("{: ^{} }", element, 6)` for adjustable width.
What you can do after
Apply it via std::format to vary alignment widths without hard-coding every format specification.
How it connects
Links replacement fields, format specifiers, field width, and std::format.
Cover of Learn C++ by Example
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