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The I/O operators: << and >> work generically on all Fusion sequences. The I/O operators are overloaded in namespace boost::fusion [9]

The operator<< has been overloaded for generic output streams such that Sequence(s) are output by recursively calling operator<< for each element. Analogously, the global operator>> has been overloaded to extract Sequence(s) from generic input streams by recursively calling operator>> for each element.

The default delimiter between the elements is space, and the Sequence is enclosed in parenthesis. For Example:

vector<float, int, std::string> a(1.0f, 2, std::string("Howdy folks!");
cout << a;

outputs the vector as: (1.0 2 Howdy folks!)

The library defines three manipulators for changing the default behavior:



Defines the character that is output before the first element.


Defines the character that is output after the last element.


Defines the delimiter character between elements.

The argument to tuple_open, tuple_close and tuple_delimiter may be a char, wchar_t, a C-string, or a wide C-string.


std::cout << tuple_open('[') << tuple_close(']') << tuple_delimiter(", ") << a;

outputs the same vector, a as: [1.0, 2, Howdy folks!]

The same manipulators work with operator>> and istream as well. Suppose the std::cin stream contains the following data:

(1 2 3) [4:5]

The code:

vector<int, int, int> i;
vector<int, int> j;

std::cin >> i;
std::cin >> set_open('[') >> set_close(']') >> set_delimiter(':');
std::cin >> j;

reads the data into the vector(s) i and j.

Note that extracting Sequence(s) with std::string or C-style string elements does not generally work, since the streamed Sequence representation may not be unambiguously parseable.

#include <boost/fusion/sequence/io.hpp>
#include <boost/fusion/include/io.hpp>

[9] __sequences__ and Views residing in different namespaces will have to either provide their own I/O operators (possibly forwarding to fusion's I/O operators) or hoist fusion's I/O operators (using declaration), in their own namespaces for proper argument dependent lookup.