如何为字符串数组实例变量动态分配空间？ [英] How do you dynamically allocate space for a string array instance variable?

问题描述

我正在尝试使用c ++创建一个简单的shell程序。我有一个带有实例变量的CommandLine类，用于参数数量和这些参数的数组。

I'm trying to create a simple shell program using c++. I have a CommandLine class with instance variables for the number of arguments and an array of those arguments.

private:
int argc;
char *argv[];

这是在CommandLine类中定义构造函数的代码：

Here is the code for the definition of my constructor in my CommandLine class:

 CommandLine::CommandLine(istream& in) {    
    string cmd;
    getline(in, cmd);
    vector<string> args;
    string arg;
    istringstream iss(cmd);
    while( iss >> arg ) args.push_back(arg);
    argc = args.size();
    argv = (char*) malloc(argc*sizeof(argv));
    }

当我尝试编译时，出现以下错误消息：

When I try to compile, I get this error message:

CommandLine.cpp：在构造函数'CommandLine :: CommandLine（std :: istream&）'中：
CommandLine.cpp：29：41：错误：分配的类型不兼容将'char *'更改为'char * [0]'

CommandLine.cpp: In constructor ‘CommandLine::CommandLine(std::istream&)’: CommandLine.cpp:29:41: error: incompatible types in assignment of ‘char*’ to ‘char* [0]’

推荐答案

将成员定义从以下位置更改：

Change your member definition from:

char * argv [];

to

char ** argv;

对于您的任务，它们将在功能上兼容。然后，以这种方式分配空间：

For your task, they will be functionally compatible. Then, allocate space this way:

argc = args.size();
argv = (char**)malloc(argc * sizeof(char*));

if(!args.empty())
{
  for(int n = 0; n < argc; ++n)
  {
    const string& src = args[n];
    char*& current = argv[n];

    current = (char*)malloc((src.length() + 1) * sizeof(char));
    memcpy(current, src.c_str(), (src.length() + 1) * sizeof(char));
  }
}

但是，我不明白为什么您不能使用Member std :: vector< string> 类型的，因为您已经使用它来临时存储命令行参数。那将是更好的解决方案（并且更不容易出错）。

However, I do not understand why you cannot use member of type std::vector<string>, since you already use it to temporarily store command line parameters. That would be much better solution (and less error-prone).

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