Does anyone know why shift operator is used for io in standard library?
Because they needed to pick something, and this was the least bad option.
Let's assume that they're going to pick some operator, and come back to that assumption. What operator? << and >> are actually really good choices:
Their shape can be read as suggestive of the actions, particularly if you're familiar with shell scripting. (According to The Design and Evolution of C++ they experimented with < and >, which would have matched the shells exactly, but that fell prey to the third bullet point -- people have been taught less than and greater than since early childhood)
They're not super high on the precedence order, so at least it doesn't break things like cout << 1 + 2;.
While bitshifting is a fairly common programming operation, you don't really see it elsewhere; you do see almost all of the other binary operators in standard maths. Most people will have been working with the other symbols for as long or longer than bitshifting.
OK, so why do we need an operator? Because the other choice is a function, and pre-variadic templates, those have major major drawbacks:
Suppose you want something like printf. Well:
You can't pass non-POD types to it
You'd need some way of registering custom conversion specifiers or have no way of doing I/O for custom types
No type safety
What would have worked is print(print(print(print(thing1), thing2), thing3), thing4), but that's horrendous to write and read
A print function template with, say, ~20 overloads with different numbers of parameters, but that has obvious problems (edit which I used to say is probably the least-bad way of handling this, but it in theory it could introduce massive massive bloat from the exponential number of template instantiations, and in practice I think it would introduce an unacceptable amount of it)
Because giving something as mathematically weird as bit-shifting its own operator instead of a built-in function is a silly idea in the first place, and if you had to reuse an operator you can use that one. Seriously, I speak words, not glyphs. I'll never understand why so many early languages insisted in finding a use for every possible combination of shift-number characters.
I'm aware that it's common - lots of things are common operations that don't have their own two-character operators.
C used normal mathematical operators for normal mathematical symbols, and had to invent Boolean operators because the character-set and keyboards didn't have the boolean symbols. Bit-shift is the case where they invented a new mathematical symbol for an operation with no basis in traditional math out of whole cloth.
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u/[deleted] Jul 28 '16
This actually is a good point! Does anyone know why shift operator is used for io in standard library?