对于openfoam或其他c++程序而言，文件的读取是尤为重要的
我们最开始学习C++时，会学到类的初始化，或者是变量定义为某个值，再对某个值进行遍历，，，
类似如下：
点击查看代码
#include // 包含头文件。using namespace std; // 指定缺省的命名空间。int main(){ int a = 10; int b = (10); int c(10); int e = {10}; int f{ 10 }; cout << "a=" << a << endl; cout << "b=" << b << endl; cout << "c=" << c << endl; cout << "e=" << e << endl; cout << "f=" << f << endl;}尤其c++，初学时候内心非常感慨，怎么这么多初始化方式，我该用哪个
现在回头看，其实能熟悉一个，其他看懂就行，看不懂都没关系，毕竟我们还有gdb或者typeid()帮忙，可以看看这个变量类型具体是什么
说到底我们要问题导向目标导向，我们学习C++是为了类似openfoam这样的大工程服务的
那学这么多初始化赋值，有必要吗
如果是个大工程或者大程序是完全没必要的，
因为大工程的数据都是从文件或者数据库中获得的
你想想，要计算一个流体问题，如果有一万个网格，手打一万个三维位置就已经非常让人头疼了，更不要说上百个组分和压力等参数了，不现实
所以又回到了问题的起点，读取文件很重要，那么如何从文件中初始化程序，如何读取文件
在看of源码之前，不妨复习下C++是怎么读取文件的，进行下知识巩固
C++多使用fstream类或ifstream类
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#include #include // ifstream类需要包含的头文件。#include // getline()函数需要包含的头文件。using namespace std;int main(){ string filename = R"(./test.txt)"; //ifstream fin(filename, ios::in); ifstream fin; fin.open(filename , ios::in); // 判断打开文件是否成功。 // 失败的原因主要有：1）目录不存在；2）文件不存在；3）没有权限，Linux平台下很常见。 if (fin.is_open() == false) { cout << "打开文件" << filename <> buffer) { cout << buffer << endl; } fin.close(); // 关闭文件，fin对象失效前会自动调用close()。 cout << "操作文件完成。\n";}OPENFOAM的文件读写，主要是用IOdictionary类
打开IOdictionary类的头文件，并不复杂
点击查看代码
class IOdictionary: public baseIOdictionary{public: // Constructors //- Construct given an IOobject IOdictionary(const IOobject&); //- Construct given an IOobject and dictionary IOdictionary(const IOobject&, const dictionary&); //- Construct given an IOobject and Istream IOdictionary(const IOobject&, Istream&); //- Copy constructor IOdictionary(const IOdictionary&); //- Move constructor IOdictionary(IOdictionary&&); //- Destructor virtual ~IOdictionary(); // Member Functions //- Is object global virtual bool global() const { return true; } //- Return complete path + object name if the file exists // either in the case/processor or case otherwise null virtual fileName filePath() const { return globalFilePath(type()); } // Member Operators //- Move assignment void operator=(IOdictionary&&);};继承于baseIOdictionary类，借助于IOobject接口有五个构造函数，这五个中一个移动构造一个拷贝构造，其他类继承可以创建自己成员函数global()和filePath()，头文件还对移动构造创建了等号赋值运算符
我们再看IOdictionary类的关键先生IOobject
IOobject就稍显复杂了，我们先看openfoam对其的描述：
IOobject defines the attributes of an object for which implicit objectRegistry management is supported, and provides the infrastructure for performing stream I/O.
An IOobject is constructed with an object name, a class name, an instance path, a reference to a objectRegistry, and parameters determining its storage status.
下面这张图是openfoam与该接口有关的类谱图
打开regIOobject类，这是个抽象类，openfoam对regIOobject的描述为：
regIOobject is an abstract class derived from IOobject to handle automatic object registration with the objectRegistry.
这里不赘述了，最后查了好半天，大概是哈希表来的，方便查询读取。openfoam中/0，/constant以及/system文件夹中那种奇怪的文件格式肯定有适合自己的读取方法。
那我们看看IOobject类相关源码，
IOobject类有四个枚举控制读取权限以及文件检查
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//- Enumeration defining the valid states of an IOobject enum objectState { GOOD, BAD }; //- Enumeration defining the read options enum readOption { MUST_READ, MUST_READ_IF_MODIFIED, READ_IF_PRESENT, NO_READ }; //- Enumeration defining the write options enum writeOption { AUTO_WRITE = 0, NO_WRITE = 1 }; //- Enumeration defining the file checking options enum fileCheckTypes { timeStamp, timeStampMaster, inotify, inotifyMaster };内共有六个构造函数，其中一个拷贝构造
点击查看代码
// Constructors //- Construct from name, instance, registry, io options IOobject ( const word& name, const fileName& instance, const objectRegistry& registry, readOption r=NO_READ, writeOption w=NO_WRITE, bool registerObject=true ); //- Construct from name, instance, local, registry, io options IOobject ( const word& name, const fileName& instance, const fileName& local, const objectRegistry& registry, readOption r=NO_READ, writeOption w=NO_WRITE, bool registerObject=true ); //- Construct from path, registry, io options // Uses fileNameComponents() to split path into components. IOobject ( const fileName& path, const objectRegistry& registry, readOption r=NO_READ, writeOption w=NO_WRITE, bool registerObject=true ); //- Construct from copy resetting registry IOobject ( const IOobject& io, const objectRegistry& registry ); //- Construct from copy resetting name IOobject ( const IOobject& io, const word& name ); //- Copy constructor IOobject(const IOobject& io) = default;我们举个例子，基于这些构造函数可以这样写类似这样的读取端口：
点击查看代码
IOobject dicName( "dicName", runTime.timeName(), //runTime.constant(), mesh, IOobject::MUST_READ, IOobject::AUTO_WRITE) ;再回到IOdictionary类，
通过查看IOdictionary类的构造函数，我们了解到可以在创建好的IOobject端口基础上构建，也可以直接构建IOobject端口
点击查看代码
IOdictionary dicName( IOobject ( "dicName", //runTime.constant(), runTime.system(), mesh, IOobject::MUST_READ, IOobject::AUTO_WRITE ) ) ;那么我们如何通过IOdictionary类读取openfoam内文件呢，
以openfoam内置icoFoam为例$ ls cavity/
0 constant system$ tree cavity/
cavity/
├── 0
│ ├── U
│ └── p
├── constant
│ ├── polyMesh
│ │ ├── boundary
│ │ ├── faces
│ │ ├── neighbour
│ │ ├── owner
│ │ └── points
│ └── transportProperties
└── system
├── blockMeshDict
├── controlDict
├── fvSchemes
└── fvSolution$ head cavity/0/* -n15
cavity/0/U *
/——————————–– C++ –———————————-
========= |
\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\ / O peration | Website: https://openfoam.org
\ / A nd | Version: 8
\/ M anipulation |
*—————————————————————————*/
FoamFile
{
version 2.0;
format ascii;
class ** volVectorField;**
object U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //cavity/0/p *
/——————————–– C++ –———————————-
========= |
\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\ / O peration | Website: https://openfoam.org
\ / A nd | Version: 8
\/ M anipulation |
*—————————————————————————*/
FoamFile
{
version 2.0;
format ascii;
class ** volScalarField;**
object p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
从以上内容可以看到，U和v分别属于volVectorField类以及volScalarField类，那么就利用IOobject接口分别创建相关类如下所示：
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volVectorField U( IOobject( "U" , runTime.timeName() , mesh , IOobject::MUST_READ , IOobject::AUTO_WRITE ) , mesh ); volScalarField p( IOobject( "p" , runTime.timeName() , mesh , IOobject::MUST_READ , IOobject::AUTO_WRITE ) , mesh );再看其他两个文件夹$ grep -rn "dicti" .
./cavity/constant/transportProperties:12: class dictionary;
./cavity/system/controlDict:12: class dictionary;
./cavity/system/fvSolution:12: class dictionary;
./cavity/system/blockMeshDict:12: class dictionary;
./cavity/system/fvSchemes:12: class dictionary;
./constant/transportProperties:12: class dictionary;
./system/controlDict:12: class dictionary;
./system/fvSolution:12: class dictionary;
./system/blockMeshDict:12: class dictionary;
./system/fvSchemes:12: class dictionary;
都是dictionary类，那创建起来更方便了，比如说contorlDict文件
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IOdictionary controlDict( IOobject ( "controlDict", runTime.constant(), //runTime.system(), mesh, IOobject::MUST_READ, IOobject::AUTO_WRITE ) ) ;有了dictionary这个字典类，顾名思义，字典当然是很方便查找啊，而且本身也是在链表以及哈希列表基础上建立的，能用的函数就很丰富了，如lookup()等。
综上，openfoam创建了一个读取方便查找容易的类支持of进行数据处理，寻根溯源很麻烦，但是用起来还是很简单的。
正因为文件读取占用很大的比重，这篇文章写了三个多小时，我们也是借此机会对openfoam内部略窥一二，这样我们就知道我们的程序在写什么，知道其所能为与不能为。
最后奉上自己编写的openfoam codesnip ，节约大家时间
点击查看代码
//***********************************************////openfoam"Foam_IOobject": {"prefix": "IOobject_Foam_","body": ["IOobject ${1|U,p,T,dicName|}(",// "\t${2|\"U\",\"p\",\"T\"|}","\t\"${1}\",","\t//runTime.timeName(),","\t//runTime.constant(),","\tmesh,","\tIOobject::MUST_READ,","\tIOobject::AUTO_WRITE",") ;"],"description": "FOAM::IOobject"},"Foam::IOdictionary": {"prefix": "IOdictionary_Foam_","body": ["IOdictionary ${1:dicName}(","\tIOobject (",// "\t${2|\"U\",\"p\",\"T\"|}","\t\t\"${1}\",","\t\t//runTime.constant(),","\t\t//runTime.system(),","\t\tmesh,","\t\tIOobject::MUST_READ,","\t\tIOobject::AUTO_WRITE","\t)",") ;"],"description": "Foam::IOdictionary"},"Foam::volVectorField": {"prefix": "volVectorField_Foam_","body": ["volVectorField ${U}(","\tIOobject (","\t\t\"${U}\",","\t\trunTime.timeName(),","\t\tmesh,","\t\tIOobject::MUST_READ,","\t\tIOobject::AUTO_WRITE","\t),","\tmesh",") ;"],"description": "Foam::volVectorField"},"Foam::volScalarField": {"prefix": "volScalarField_Foam_","body": ["volScalarField ${p}(","\tIOobject (","\t\t\"${p}\",","\t\trunTime.timeName(),","\t\tmesh,","\t\tIOobject::MUST_READ,","\t\tIOobject::AUTO_WRITE","\t),","\tmesh",") ;"],"description": "Foam::volScalarField"},"Foam::scalar": {"prefix": "scalar","body": ["scalar "],"description": "Foam::scalar"},//***********************************************//创作不易，如若喜欢，不胜感激，欢迎支持，欢迎指正