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This article mainly introduces "what is the C++ standard library and std namespace". In daily operation, I believe many people have doubts about what is the C++ standard library and std namespace. Xiaobian consulted all kinds of information and sorted out simple and easy to use operation methods. I hope to help you answer the doubts of "what is the C++ standard library and std namespace"! Next, please follow the small series to learn together!

C++ was developed on the basis of the C language. The early C++ was not perfect, did not support namespaces, did not have its own compiler, but translated C++ code into C code, and then compiled through the C compiler. C++ at this time is still using C language libraries, stdio.h, stdlib.h, string.h and other header files are still valid; in addition, C++ has also developed some new libraries, adding its own header files, such as:

iostream.h: Used for console I/O header files.

fstream.h: Header file used for file operations.

complex.h: Header file used for complex computations.

Like C, C++ header files continue to have.h suffixes, and they contain classes, functions, macros, etc. that are globally scoped.

Later, C++ introduced the concept of namespace, and planned to rewrite the library to integrate classes, functions, macros, etc. into a namespace named std. Std is an abbreviation for standard, meaning "standard namespace."

But there were already many programs written in old-style C++ that didn't use namespaces in their code, and modifying the original library directly had a serious consequence: programmers would resist using the new standard C++ code because they didn't want to spend a lot of time modifying the old-style code.

C++ developers think of a good way to keep the original library and header files, they can continue to use in C++, and then copy the original library, on this basis, slightly modified, classes, functions, macros, etc. into the namespace std, it becomes a new version of the C++ standard library. Thus, there are two libraries with similar functions, programs that use the old C++ can continue to use the original library, and newly developed programs can use the new C++ library.

In order to avoid duplicate header names, the new C++ library has also changed the naming of header files, removing the suffix.h, so that the old C++ iostream.h becomes iostream and fstream.h becomes fstream. For the original C language header file, the same method is used, but a c letter is added before each name, so the C language stdio.h becomes cstdio, and stdlib.h becomes cstdlib.

Note that the old C++ headers are officially deprecated and have been explicitly deprecated, but the old C headers can still be used to maintain compatibility with C. In fact, compiler developers will not stop providing support for existing customer software, and it can be expected that old C++ headers will be supported for years to come.

Here is my summary of the C++ header file status:

1)Old C++ header files such as iostream.h, fstream.h, etc. will continue to be supported, although they are not in the official standard. The contents of these header files are not in namespace std.

2)The new C++ header files, such as iostream, fstream, etc., contain basic functionality similar to the corresponding old header files, but the contents of the header files are in the namespace std.

Note: During standardization, details of some parts of the library were modified, so the old header file and the new header file may not correspond exactly.

3)Standard C header files such as stdio.h, stdlib.h, etc. continue to be supported. The contents of the header file are not in std.

4)New C++ header files with C library functionality have names like cstdio, cstdlib. They provide the same content as the corresponding old C header file, except that the content is in std.

It can be seen that for header files without.h, all symbols are in namespace std, which needs to be declared when used; for header files with.h, no namespace is used, and all symbols are in global scope. This is also defined by the C++ standard.

However, the reality is somewhat different from what the C++ standard expects. For the original C language header file, even if it is used in the C++ way, i.e.#include, then the symbol can be located in the namespace std or in the global scope. Please see the following two paragraphs of code.

1)Use namespace std:

#include int main(){std::printf("http://c.biancheng.net\n");return 0;}

2)No namespace std:

#include int main(){printf("http://c.biancheng.net\n");return 0;}

These two forms can be compiled under Microsoft Visual C++ and GCC, that is, most compilers do not strictly follow the standard when implemented, they support both writing, programmers can use std or not.

Although they are error-free in current compilers, I still recommend using type 1) because standard writing will always be supported by compilers, and non-standard writing may not be supported in future upgrades.

Use C++ header files

Although C++ is almost fully compatible with C, C language header files are still supported in C++, but C++ new libraries are more powerful and flexible, please try to use these C++ new header files, such as iostream, fstream, string, etc.

In the previous sections, we used the C language format output function printf, introduced the C language header file stdio.h, and mixed C code with C++ code. I don't recommend doing this, please try to use C++. The following example demonstrates how to use C++ libraries for input and output:

#include #include int main(){//declare namespace stdusing namespace std;//define string variable str;//define int variable int age;//get user input from console cin>>str>>age;//output data to console cout

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