将字符串内容用于CPP中的另一个程序

Question

我正在尝试将从一个程序读取的字符串内容用于另一个程序。我已经放了一些示例代码来重现这个例子。我有一个main函数

//main.cpp
#include "read.h"
#include "read_fasta.h"
#include <iostream>
#include <string>

int main(int argc, char **argv)

{
    index();
    read_fasta_file(argc,argv); 

    std::cout << " : " << content << std::endl;
    std::cout << "Length of fasta file: " << content.length() << std::endl;

    return 0;
}

我看了fasta档

//read_fasta.cpp
#include <iostream>
#include <fstream>
#include "read_fasta.h"

std::string content;

int read_fasta_file(int argc, char **argv)
 {
     std::ifstream input(argv[1]);
     std::string line, name;
     while( std::getline( input, line ).good() ){
           if( line.empty() || line[0] == '>' ){ // Identifier marker
             if( !name.empty() ){ // Print out what we read from the last entry
                 std::cout << name << " : " << content << std::endl;
                    name.clear();
                }
                if( !line.empty() ){
                    name = line.substr(1);
                }
                content.clear();
            } else if( !name.empty() ){
                if( line.find(' ') != std::string::npos ){ // Invalid sequence--no spaces allowed
                    name.clear();
                    content.clear();
                } else {
                    content += line;
                }
            }
        }
        if( !name.empty() ){ // Print out what we read from the last entry
            std::cout << name << " : " << content << std::endl;
        std::cout << "Length of fasta file: " << content.length() << std::endl;
        }   
        return 0;

    }

read_fasta.cpp

的标头文件

//read_fasta.h
#ifndef read_fasta_h
#define read_fasta_h
#include <iostream>
#include <string>
#include <fstream>

int read_fasta_file(int argc, char **argv);

extern std::string content;

#endif

直到现在每件事都运转良好。现在，我尝试在另一个std::string content程序中使用cpp。

//read.cpp
#include <iostream>
#include <string>
#include "read.h"

void index() 
{

std::cout<<"Do U see any printing :"<<content<<std::endl;

}

read.cpp

的标头文件

//read.h
#ifndef read_h
#define read_h

#include <iostream>
#include <string>
#include <vector>

void index(); 
extern std::string content;

#endif

但std::string content计划中的read.cpp似乎没有任何内容。我错过了什么。