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Showing content from https://cplusplus.com/reference/functional/placeholders/ below:

namespace

std::placeholders

namespace placeholders {  extern /* unspecified */ _1;  extern /* unspecified */ _2;  extern /* unspecified */ _3;  // ...}

Bind argument placeholders

This namespace declares an unspecified number of objects: _1, _2, _3,..., which are used to specify placeholders in calls to function bind.

When the function object returned by bind is called, an argument with placeholder _1 is replaced by the first argument in the call, _2 is replaced by the second argument in the call, and so on... For example:

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using namespace std::placeholders;
auto bound_fn = std::bind (fn,100,_1);
bound_fn(5);  // calls fn(100,5), i.e.: replacing _1 by the first argument: 5

The type of these placeholder objects is unspecified (it depends on the library implementation, see is_placeholder), but in all cases their type shall at least be nothrow default-constructible and nothrow copy-constructible. Whether assignment operations or additional constructors are supported is implementation-defined, but any copy-assignment or move-constructor shall also be not throwing.

When a call to bind is used as a subexpression in another call to bind, the placeholders are relative to the outermost bind expression.

Objects

_1: Replaced by the first argument in the function call.
_2: Replaced by the second argument in the function call.
_N: Replaced by the Nth argument in the function call (where N is a natural number).