C++:从函数访问主变量的最简单方法

我目前正在处理一个在main（）中初始化的字符串，如果我尝试将其全局化，由于某种原因，它会变为非字符字符串。我想知道是否可以让程序中声明的函数访问这个变量。。。函数本身将只在main或从main调用的函数中执行，但编译器在到达main（）之前不会看到变量名。

以下是相关代码的精简版本：

string getCommand(int input_pos,string inputstring)
{
int temp_paren=0;
int begin_pos = stdinstring.rfind("(",input_pos);
int len = 0;
while (temp_paren>0 && len < 10)
 {
 if (stdinstring.substr(begin_pos+len,1)=="(") {temp_paren++;}
 if (stdinstring.substr(begin_pos+len,1)==")") {temp_paren--;}
 len++;
 }
 return stdinstring.substr(begin_pos,len);
}
int main(void) {
string stdinstring = ""; 
}

我一直在考虑前向声明，每次使用时手动将变量传递给函数，甚至创建一个类来保存该变量。。。解决这个问题最简单的方法是什么？理想情况下，我会有足够的时间将其作为一个全局变量，并弄清楚哪里出了问题/为什么出了问题，但我只需要完成这项工作。谢谢你的帮助。

编辑：对于任何感兴趣的人：如果我尝试使字符串成为全局字符串，就会发生这种情况。所有字符都变成了"（如果可见，则不知道）

编辑：好的，这是我的代码的一个更完整的版本。此外，我在上面尝试粘贴的符号是带有"0 0 0 1"（ASCII字符代码？）的方框

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<stdint.h>
//#include<regex>
#define PI M_PI
#define VERBOSE 1
using namespace std;
string stdinstring; 
template <class dataclass> 
struct linkm {
  dataclass value;
  linkm *next;
};
template <class dataclass> 
class linklist 
{
  public:
    linklist()
      {top = NULL;}
    ~linklist() 
      {}
    void push(dataclass num)
      {
      linkm<dataclass> *temp = new linkm<dataclass>;
      temp->value = num;
      temp->next = top;
      top = temp;
      } 
    dataclass pop()
      {
      if (top == NULL) 
        return 0;
      linkm<dataclass> * temp;
      temp = top;
      dataclass value;
      value = temp->value;
      top = temp->next;
      delete temp;
      return value;
      }
    bool isEmpty()
      {
      if (top == NULL) 
        return 1;
      return 0;
      }
  private:
    linkm<dataclass> *top; 
};
double evaluateExpression(string expression)
{
// sample expression : (* (/ a 1) (+ b 2))
     if (expression.substr(0,2)=="(+") {cout << "add found"<<endl;}
else if (expression.substr(0,2)=="(-") {cout << "sub found"<<endl;}
else if (expression.substr(0,2)=="(*") {cout << "mult found"<<endl;}
else if (expression.substr(0,2)=="(/") {cout << "div found"<<endl;}
else if (expression.substr(0,2)=="(sin") {cout << "sin found"<<endl;}
else if (expression.substr(0,2)=="(cos") {cout << "cos found"<<endl;}
else {cout << "Error: invalid operation";} // or is it just a number?
}
string getCommand(int input_pos,string inputstring)
{
int temp_paren=0;
int begin_pos = stdinstring.rfind("(",input_pos);
int len = 0;
while (temp_paren>0 && len < 10)
 {
 if (stdinstring.substr(begin_pos+len,1)=="(") {temp_paren++;}
 if (stdinstring.substr(begin_pos+len,1)==")") {temp_paren--;}
 len++;
 }
 return stdinstring.substr(begin_pos,len);
}
class symContainer
{
public:
string index[500];
float value[500];
int currindex;
symContainer () { currindex = 0 ; }
void add(string id,float invalue)
{
index[currindex] = id;
value[currindex] = invalue;
currindex++;
}
float get(string id)
{
int i=0;
while (i<currindex)
 {
 if(id==index[i]) {return value[i];}
 i++;
 }
 cout << "Invalid input - an unassigned symbol was requested:" << id << endl;
 exit(2);
}
};

struct transform { int type; double arguments[4]; } ;
struct point { double x; double y; } ;
struct drawing { linklist<point> points; linklist<transform> transforms; void applyTransforms(linklist<transform> * trsptr) {}  } ; 
struct group : public drawing { linklist<int> drawings; void transform(linklist<transform> * trsptr) {} } ;
struct symbol {string index; double value;};
char getNext() { //for the lookaround function. has hard-filters, like replacing newlines/tabs with spaces
char temp = getchar(); 
if (temp=='n') {temp=' ';}
if (temp=='t') {temp=' ';}
return temp;
}
int main(void) {
stdinstring="a";
char command[20], args[2048];
int commandindex=0;     //'i' for what command we're on
int stdinsize=2;
double argsArray[8];
bool filled=0;
int parenlevel = 0;
symContainer symbol;
//1 is the character that will be written at the end of each loop. 0 and 2 are 1 char ahead/behind, respectively
char c_lookaround[2]; 
c_lookaround[0]=NULL;
c_lookaround[1]=getNext();
c_lookaround[2]=getNext();
unsigned long i=0;
bool write=0;
while( c_lookaround[2] != EOF )
{ 
write=1;
// Lookaround logic goes here. (Clearing duplicate whitespaces, newlines, and the like)
while (                         c_lookaround[1]==' '  && c_lookaround[2]==' ' ) {c_lookaround[2]=getNext();}
while (c_lookaround[0]=='('  && c_lookaround[1]==' '                          ) {c_lookaround[1]=c_lookaround[2]; c_lookaround[2]=getNext();}
while (                         c_lookaround[1]==' '  && c_lookaround[2]==')' ) {c_lookaround[1]=c_lookaround[2]; c_lookaround[2]=getNext();}
while (c_lookaround[0]==NULL && c_lookaround[1]==' '                          ) {c_lookaround[1]=c_lookaround[2]; c_lookaround[2]=getNext();}
//while (c_lookaround[0]==')'  && c_lookaround[1]==' '  && c_lookaround[2]=='(' ) {c_lookaround[1]=c_lookaround[2]; c_lookaround[2]=getNext();}
//while (c_lookaround[0]==')'  && c_lookaround[1]==' '  && c_lookaround[2]=='' ) {cout<<"aa";c_lookaround[1]=c_lookaround[2]; c_lookaround[2]=getNext();}
if (c_lookaround[0]=='('  && c_lookaround[1]==':'  && c_lookaround[2]=='=') 
 {
    getCommand(i,stdinstring);
 }
//Determine current parentheses level
if (c_lookaround[1] == '(') { parenlevel++;}
if (parenlevel==0) {write=0;}
if (c_lookaround[1] == ')') { parenlevel--;}
//Write the character 
if (write) {stdinstring.push_back(c_lookaround[1]);}
cout << stdinstring<< endl;
//Advance the tape!
i++;
c_lookaround[0]=c_lookaround[1];
c_lookaround[1]=c_lookaround[2];
c_lookaround[2]=getNext();
}
stdinsize = i;
}