3 ms·

It's crazy to see how much work header files make the C++ compiler do just to build hello world: $ gcc -E hello.c | cloc --force-lang=c - 419 $ g++

by gpu64 3y ago

It's crazy to see how much work header files make the C++ compiler do just to build hello world: $ gcc -E hello.c | cloc --force-lang=c - 419 $ g++ -E -std=c++11 hello_iostream.cpp | cloc --force-lang=c++ - 20707 $ g++ -E -std=c++20 hello_iostream.cpp | cloc --force-lang=c++ - 30876 $ g++ -E -std=c++20 hello_std_format.cpp | cloc --force-lang=c++ - 42757 Just bumping the C++ std version can add thousands of lines of code.

nly 3y ago
iostreams are more or less just a straight conversion of everything that C streams were (but nobody ever used) in to polymorphic interfaces and templates. It's a shame they add so much header bloat, but there's a lot of functionality in there and it's hard to see a way it could have been avoided if you want type safety.
- leni536 3y ago
  Most of the sin of iostream is having a humonguous overload set for operator<<. AFAIK it's not too bad to just compile the stream headers, but the overload resolution for `std::cout << anything` is expensive.
  - phaedrus 3y ago
    And after all that effort, an operator overload is still the "wrong" API for the job anyway, because: it's missing a crucial argument! Any binary (infix) operator can only relate two things, but typically when you want to print something you actually want to relate three things: the stream, the object or value to print, and the way you want values represented. Oh, sure, you can manipulate this by inserting magic formatting objects, but that's not different from saying all functions take one argument a la Category Theory or Haskell without syntactic sugar. Or you could write a class or function that allows you to tag your object or value by changing its type to a different one so a different operator<< overload is called. In the iostreams design, you're only indirectly selecting which print function to use by manipulating either the stream or the printed object. But in actuality the types of your two arguments are not different, so either you must munge global settings on the stream or "hack" which operator<< overload is selected. And that's the crux of the issue; what you "really want" is a stream API with three arguments: the stream, the object or value, and a pointer to a function (or function object) that takes the value and transforms it to a stream of characters. Why not just pass that function directly and explicitly? Of course that requires something that's not just a single chained infix operator; my thesis being that starting with that quirky choice leads to down a path that ends in a bad design.
    - bregma 3y ago
      > what you "really want" is a stream API with three arguments: the stream, the object or value, and a pointer to a function (or function object) that takes the value and transforms it to a stream of characters. Or maybe a format object that returns a string in the way described by some directive, the way printf() worked in C or format conversions work in Python? You could call it something like std::format and make it available through a standard header like <format>.
- pjmlp 3y ago
  Also taking us back to 1990's, anyone coming from Object Pascal, Smalltalk or Common Lisp would feel right at home with iostreams architecture. As for the header bloat, fear not, import std; for the complete C++ standard library, is faster than a plain #include <iostream>, as per VC++ team measurements.
menaerus 3y ago
> Just bumping the C++ std version can add thousands of lines of code. And yet none of this heavily influences compile times. At least I have not found so on MM LoC projects I've been working so far. Not sure what the fuss is around modules, I honestly don't think it will solve C++ build times. Happy to be proved otherwise.
- comex 3y ago
  Try compiling a small source file that includes a handful of STL headers, and compare to one that doesn’t. Even just including without instantiating any of the templates, the difference is a substantial fraction of a second. That can be a heavy influence, though maybe not in a huge codebase which also has plenty of large headers of its own.
  - menaerus 3y ago
    I tried couple of times to workaround the problem you're describing through PCH on some of my ex projects. All of them were on the MM LoC scale and build-from-scratch was taking roughly 45min to 1hr on a developer machine. I never got to make a substantial cut in the build time through PCH, only perhaps by a very few % but YMMV. Given that modules are PCH in disguise, I don't believe modules are going to have a dramatically bigger, and better, effect on C++ build times. Whether or not they are going to cut the build time from 45 seconds to 30 seconds I also couldn't care less - small projects don't even have the build time problem worth solving. Mid and big projects is where it will really count and if modules are going to have a substantial effect in the range of at least 20-30% in build time cut, I will be all ears.
    - gpu64 3y ago
      Yeah, how much modules/PCHs would help probably depends a lot on your code base. I read about clangs work on modules and they seemed a bit uncertain if you'd get much benefit at all once you switch on optimizations. https://clang.llvm.org/docs/StandardCPlusPlusModules.html#how-modules-speed-up-compilation https://clang.llvm.org/docs/StandardCPlusPlusModules.html#ho...
lallysingh 3y ago
How many of those lines are blank after #ifdef?
- gpu64 3y ago
  I used cloc so blank lines and comments should be ignored.
tovej 3y ago
I don't really see many codebases using iostreams anymore, it's all either cstdio or fmt

by gpu64 3y ago

nly 3y ago

iostreams are more or less just a straight conversion of everything that C streams were (but nobody ever used) in to polymorphic interfaces and templates. It's a shame they add so much header bloat, but there's a lot of functionality in there and it's hard to see a way it could have been avoided if you want type safety.

leni536 3y ago
Most of the sin of iostream is having a humonguous overload set for operator<<. AFAIK it's not too bad to just compile the stream headers, but the overload resolution for `std::cout << anything` is expensive.
- phaedrus 3y ago
  And after all that effort, an operator overload is still the "wrong" API for the job anyway, because: it's missing a crucial argument! Any binary (infix) operator can only relate two things, but typically when you want to print something you actually want to relate three things: the stream, the object or value to print, and the way you want values represented. Oh, sure, you can manipulate this by inserting magic formatting objects, but that's not different from saying all functions take one argument a la Category Theory or Haskell without syntactic sugar. Or you could write a class or function that allows you to tag your object or value by changing its type to a different one so a different operator<< overload is called. In the iostreams design, you're only indirectly selecting which print function to use by manipulating either the stream or the printed object. But in actuality the types of your two arguments are not different, so either you must munge global settings on the stream or "hack" which operator<< overload is selected. And that's the crux of the issue; what you "really want" is a stream API with three arguments: the stream, the object or value, and a pointer to a function (or function object) that takes the value and transforms it to a stream of characters. Why not just pass that function directly and explicitly? Of course that requires something that's not just a single chained infix operator; my thesis being that starting with that quirky choice leads to down a path that ends in a bad design.
  - bregma 3y ago
    > what you "really want" is a stream API with three arguments: the stream, the object or value, and a pointer to a function (or function object) that takes the value and transforms it to a stream of characters. Or maybe a format object that returns a string in the way described by some directive, the way printf() worked in C or format conversions work in Python? You could call it something like std::format and make it available through a standard header like <format>.
pjmlp 3y ago
Also taking us back to 1990's, anyone coming from Object Pascal, Smalltalk or Common Lisp would feel right at home with iostreams architecture. As for the header bloat, fear not, import std; for the complete C++ standard library, is faster than a plain #include <iostream>, as per VC++ team measurements.

menaerus 3y ago

> Just bumping the C++ std version can add thousands of lines of code. And yet none of this heavily influences compile times. At least I have not found so on MM LoC projects I've been working so far. Not sure what the fuss is around modules, I honestly don't think it will solve C++ build times. Happy to be proved otherwise.

comex 3y ago
Try compiling a small source file that includes a handful of STL headers, and compare to one that doesn’t. Even just including without instantiating any of the templates, the difference is a substantial fraction of a second. That can be a heavy influence, though maybe not in a huge codebase which also has plenty of large headers of its own.
- menaerus 3y ago
  I tried couple of times to workaround the problem you're describing through PCH on some of my ex projects. All of them were on the MM LoC scale and build-from-scratch was taking roughly 45min to 1hr on a developer machine. I never got to make a substantial cut in the build time through PCH, only perhaps by a very few % but YMMV. Given that modules are PCH in disguise, I don't believe modules are going to have a dramatically bigger, and better, effect on C++ build times. Whether or not they are going to cut the build time from 45 seconds to 30 seconds I also couldn't care less - small projects don't even have the build time problem worth solving. Mid and big projects is where it will really count and if modules are going to have a substantial effect in the range of at least 20-30% in build time cut, I will be all ears.
  - gpu64 3y ago
    Yeah, how much modules/PCHs would help probably depends a lot on your code base. I read about clangs work on modules and they seemed a bit uncertain if you'd get much benefit at all once you switch on optimizations. https://clang.llvm.org/docs/StandardCPlusPlusModules.html#how-modules-speed-up-compilation https://clang.llvm.org/docs/StandardCPlusPlusModules.html#ho...

lallysingh 3y ago

How many of those lines are blank after #ifdef?

gpu64 3y ago
I used cloc so blank lines and comments should be ignored.

tovej 3y ago

I don't really see many codebases using iostreams anymore, it's all either cstdio or fmt