在C ++ Ubuntu Linux中运行Shell命令时出错

Question

//all variables are declared in a struct StockPile
//...
string itemid;    
string itemdesc;  
string datepurchased;
string line;
int unitprice;
int totalsales;
std::string myline;
//...

void displaydailyreport() {

    ifstream myfile("stockdatabase.txt");   

    for(int i=0;std::getline(myfile,myline);i++) 
    {
        // Trying to grep all data with a specific date from a textfile,
        cout<<system("grep "<<stockpile[i].datepurchased<<" stockdatabase.txt")<<endl;
    }   
    cout<<endl; 
}

When I try to compile it gives me this error :

note:template argument deduction/substitution failed:
Main.cpp:853:40: note:   mismatched types ‘std::basic_ostream<_CharT, _Traits>’ and ‘const char [6]’
     cout<<system("grep "<<stockpile[i].datepurchased<<" stockdatabase.txt")<<endl;

When I try to run with this it works fine :

 cout<<system("grep '9oct16' stockdatabase.txt")

stockpile[i].datepurchased is where I can cout the different dates stored in my textfile , I can print out stockpile[i].datepurchased values in the for loop. It returns string 9oct16 , 10oct16 etc. but when I try to use shell command it wont compile .

Answer 1

The << operator is a stream operator. While you can concatenate strings (and c-strings) with them on a stream (like cout ) it does not work that way without actually working on a stream.

Lets just take the statement inside your system call separately

"grep "<<stockpile[i].datepurchased<<" stockdatabase.txt"

The << is not meant to be used that way without a stream object to "stream" into.

What you can do though is the following:

std::string command = "grep "
                      + stockpile[i].datepurchased
                      + " stockdatabase.txt"
system(command.c_str());

This does several things.

• create a std::string to store the system command
• because datepurchased is a std::string already you can use the + operator on the other c-strings to concatenate them.
• system is expecting a const char* as argument. So to be able to pass the c-string to the function we use the c_str() function of the std::string

You can also shorten the statement to this:

system( ("grep "+stockpile[i].datepurchased+" stockdatabase.txt").c_str());

Because a temporary std::string will be created by the + operator you can access its c_str() function directly.

Answer 2

<< is an operator to append to a stream, it does not do string concatenation. So please use + instead of << to generate the grep command.

http://www.cplusplus.com/reference/string/string/operator+/

http://www.cplusplus.com/reference/string/string/operator%3C%3C/

Answer 3

This is wrong:

cout<<system("grep "<<stockpile[i].datepurchased<<" stockdatabase.txt")<<endl

Your need a stringstream object to stream the parts of the command first. Or build the command using string like that:

std::string command = "grep ";
command +=stockpile[i].datepurchased;
command +=" stockdatabase.txt";

cout<<system( command.c_str() )<<endl

Answer 4

Well, your compiler quite clearly tells you what's wrong: You are trying to use the stream operator << with two non-matching types, namely const char [6] (aka "grep " ) and a std::ostream (aka stockpile[i].datepurchased ). You simply cannot stream into char arrays or strings. That's what streams are designed for in STL. So one possible solution could be:

std::cout << system((std::string("grep ") + stockpile[i].datepurchased + std::string(" stockdatabase.txt")).c_str()) << std::endl;

Didn't test it, though ;)