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## Struct display_expr

boost::proto::functional::display_expr — Pretty-print a Proto expression tree.

## Synopsis

    // In header: <boost/proto/debug.hpp>

    struct display_expr {
      // types
      typedef void result_type;

      // construct/copy/destruct
      display_expr(std::ostream & = std::cout, int = 0);

      // public member functions
      template<typename Expr> void operator()(Expr const &) const;
    };

## Description

A PolymorphicFunctionObject which accepts a Proto expression tree and pretty-prints it to an `ostream` for debugging purposes.

### `display_expr` public construct/copy/destruct

1.     display_expr(std::ostream & sout = std::cout, int depth = 0);

   Parameters: |
   `sout` | The `ostream` to which the expression tree will be written.
   `depth` | The starting indentation depth for this node. Children nodes will be displayed at a starting depth of `depth+4`.

### `display_expr` public member functions

1.     template<typename Expr> void operator()(Expr const & expr) const;