stringstream and binary data

Question

What is needed (some method overrides?) in order to read/write binary data to/from std::basic_stringstream? I am trying the following code, but it does not work as I supposed:

std::basic_stringstream<uint64_t> s;
uint64_t a = 9;
s << a;
uint64_t b;
s >> b;
std::cout << b << std::endl;

but I get "0" printed (built with GCC).

There is no built-in specialization of std::char_traits<uint64_t>. You won't be able to instantiate std::basic_stringstream<uint64_t> until you provide one. Now, what is it you are trying to achieve? Your code doesn't make much sense to me. It prints 9 if you just use plain vanilla std::stringstream, but there's no "binary data" in there, and it's not clear what you mean by that. — Igor Tandetnik
– Igor Tandetnik, Commented Nov 5, 2017 at 19:02
no offense, but afaik basic_stringstream is for those that know what they are doing. What do you actually want to achieve? I am almost sure that basic_stringstream isnt the right tool for it — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Nov 5, 2017 at 19:05
Are you trying to convert a 64-bit decimal number to a textual binary representation, such as "1001" for 9? — Thomas Matthews
– Thomas Matthews, Commented Nov 5, 2017 at 19:08

Galik · Accepted Answer · 2017-11-05 19:10:59Z

18

If you want to read/write binary data you can't use << or >> you need to use the std::stringstream::read and std::stringstream::write functions.

Also you need to use the <char> specialization because only char can safely alias other types.

So you could do it this way:

std::stringstream ss;

std::uint64_t n1 = 1234567890;

ss.write((char const*) &n1, sizeof(n1)); // sizeof(n1) gives the number of char needed

std::uint64_t n2;

ss.read((char*) &n2, sizeof(n2));

std::cout << n2 << '\n';

Output:

1234567890

answered Nov 5, 2017 at 19:10

Galik

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2 Comments

Tellegar Over a year ago

why can't you use << >> with binary stream?

Galik Over a year ago

@Tellegar Both << and >> are formatting operations - that modify/change the data being read or written. Binary streams need to be transmitted and received unchanged.

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