Compiling a C++ file takes a very long time when compared to C#, Java. It takes significantly longer to compile a C++ file than it would to run a normal size Python script. I'm current using VC++ but it's the same with any compiler. Why is this?
The two reasons I could think of were loading header files and running the preprocessor, but that doesn't seem like it should explain why it takes so long.
Some reasons are:
1) C++ grammar is more complex than C# or Java and takes more time to parse.
2) (More important) C++ compiler produces machine code and does all optimizations during compilation. C# and Java go just half way and leave these steps to JIT.
C++ is compiled into machine code. So you have the pre-processor, the compiler, the optimizer, and finally the assembler, all of which have to run.
Java and C# are compiled into byte-code/IL, and the Java virtual machine/.NET Framework execute (or JIT compile into machine code) prior to execution.
Python is an interpreted language that is also compiled into byte-code.
I'm sure there are other reasons for this as well, but in general, not having to compile to native machine language saves time.
Another reason is the use of the C pre-processor for locating declarations. Even with header guards, .h still have to be parsed over and over, every time they're included. Some compilers support pre-compiled headers that can help with this, but they are not always used.
See also: C++ Frequently Questioned Answers
A compiled language is always going to require a bigger initial overhead than an interpreted language. In addition, perhaps you didn't structure your C++ code very well. For example:
#include "BigClass.h"
class SmallClass
{
BigClass m_bigClass;
}
Compiles a lot slower than:
class BigClass;
class SmallClass
{
BigClass* m_bigClass;
}
Several reasons:
Header files: Every single compilation unit requires hundreds or even thousands of headers to be 1: loaded, and 2: compiled. Every one of them typically has to be recompiled for every compilation unit, because the preprocessor ensure that the result of compiling a header might vary between every compilation unit. (A macro may be defined in one compilation unit which changes the content of the header).
This is probably the main reason, as it requires huge amounts of code to be compiled for every compilation unit, and additionally, every header has to be compiled multiple times (once for every compilation unit that includes it)
Linking: Once compiled, all the object files have to be linked together. This is basically a monolithic process that can't very well be parallelized, and has to process your entire project.
Parsing: The syntax is extremely complicated to parse, depends heavily on context, and is very hard to disambiguate. This takes a lot of time
Templates: In C#, List<T> is the only type that is compiled, no matter how many instantiations of List you have in your program. In C++, vector<int> is a completely separate type from vector<float>, and each one will has to be compiled separately.
Add to this that templates make up a full turing-complete "sub-language" that the compiler has to interpret, and this can become ridiculously complicated. Even relatively simple template metaprogramming code can define recursive templates that create dozens and dozens of template instantiations. Templates may also result in extremely complex types, with ridiculously long names, adding a lot of extra work to the linker. (It has to compare a lot of symbol names, and if these names can grow into many thousand characters, that can become fairly expensive).
And of course, they exacerbate the problems with header files, because templates generally have to be defined in headers, which means far more code has to be parsed and compiled for every compilation unit. In plain C code, a header typically only contains forward declarations, but very little actual code. In C++, it is not uncommon for almost all the code to reside in header files.
Optimization: C++ allows some very dramatic optimizations. C# or Java don't allow classes to be completely eliminated (they have to be there for reflection purposes), but even a simple C++ template metaprogrogram can easily generate dozens or hundreds of classes, all of which are inlined and eliminated again in the optimization phase.
Moreover, a C++ program must be fully optimized by the compiler. A C# program can rely on the JIT compiler to perform additional optimizations at load-time, C++ doesn't get any such "second chances". What the compiler generates is as optimized as it's going to get.
Machine code: C++ is compiled to machine code which may be somewhat more complicated than the bytecode Java or .NET use (especially in the case of x86).
(This is mentioned out of completeness only because it was mentioned in comments and such. In practice, this step is unlikely to take more than a tiny fraction of the total compilation time.)
Most of these factors are shared by C code, which actually compiles fairly efficiently. The parsing step is a lot more complicated in C++, and can take up significantly more time, but the main offender is probably templates. They're useful, and make C++ a far more powerful language, but they also take their toll in terms of compilation speed.
The slowdown is not necessarily the same with any compiler.
I haven't used Delphi or Kylix but back in the MS-DOS days, a Turbo Pascal program would compile almost instantaneously, while the equivalent Turbo C++ program would just crawl.
The two main differences were a very strong module system and a syntax that allowed single-pass compilation.
It's certainly possible that compilation speed just hasn't been a priority for C++ compiler developers, but there are also some inherent complications in the C/C++ syntax that make it more difficult to process. (I'm not an expert on C, but Walter Bright is, and after building various commercial C/C++ compilers, he created the D language. One of his changes was to enforce a context-free grammar to make the language easier to parse.)
Also, you'll notice that generally Makefiles are set up so that every file is compiled separately in C, so if 10 source files all use the same include file, that include file is processed 10 times.
Parsing and code generation are actually rather fast. The real problem is opening and closing files. Remember, even with include guards, the compiler still have open the .H file, and read each line (and then ignore it).
A friend once (while bored at work), took his company's application and put everything -- all source and header files-- into one big file. Compile time dropped from 3 hours to 7 minutes.
The trade off you are getting is that the program runs a wee bit faster. That may be a cold comfort to you during development, but it could matter a great deal once development is complete, and the program is just being run by users.
The biggest issues are:
1) The infinite header reparsing. Already mentioned.
2) The fact that the toolchain is often separated into multiple binaries (make, preprocessor, compiler, assembler, archiver, impdef, linker, and dlltool in extreme cases) that all have to reinitialize all the time for each (preprocessor, compiler, assembler) or every couple of files (archiver, linker, and dlltool).
See also this discussion on comp.compilers: http://compilers.iecc.com/comparch/article/03-11-078 specially this one:
http://compilers.iecc.com/comparch/article/02-07-128
Note that John, the moderator of comp.compilers seems to agree, and that this means it should be possible to achieve similar speeds for C too, if one integrates the toolchain fully and implements precompiled headers. Many commercial C compilers do this to some degree.
Most answers are being a bit unclear in mentioning that C# will always run slower due to the cost of performing actions that in C++ are performed only once at compile time, this performance cost is also impacted due runtime dependencies (more things to load to be able to run), not to mention that C# programs will always have higher memory footprint, all resulting in performance being more closely related to the capability of hardware available. The same is true to other languages that are interpreted or depend on a VM.