错误：&#39;operator&lt;&lt;&#39; 不匹配（操作数类型是 &#39;std::ifstream&#39; {aka &#39;std::basic_ifstream<char> &#39;} 和 &#39;CHAR [39]&#39; {aka &#39;char [39]&#39;})|

Question

i am trying to look if a file name hardware id txt exist, if not then make it and write the hardware id in it, however i keep getting this error that claims that there is no much to an operator.

#include <stdio.h>
#include <iostream>
#include <fstream>

using namespace std;

int main()
{

    HW_PROFILE_INFO hwProfileInfo; // unless we have a very good reason, this should
                                   // not be global
    if(GetCurrentHwProfile(&hwProfileInfo) != NULL)
    { // update info was a success. NOW we have a GUID and can do stuff with
      // hwProfileInfo
        printf("Hardware GUID: %s\n",     hwProfileInfo.szHwProfileGuid);
        printf("Hardware Profile: %s\n", hwProfileInfo.szHwProfileName);
        std::ifstream hwidfile;
        hwidfile.open("hardwareid2.txt");
        if(hwidfile) {
            printf("File exist");
            return 0;
        } else{
            std::ofstream hwidfile { "hardwareid2.txt" };
        }

        if (!(hwidfile << hwProfileInfo.szHwProfileGuid))
        { // will fail if can't write for any reason, like file didn't open
            std::cout << "File write failed\n";
            return -1;
        }


    }
    else
    {
        std::cout << "GetCurrentHwProfile failed\n";
        return -1;
    }

    getchar();
}

Answer 1

You are declaring two separate variables, one std::ifstream and one std::ofstream , with the same name hwidfile . That won't work in your case. You are declaring the ofstream in a nested scope that destroyed the ofstream before you try to write to it. You can't change a variable's type by re-declaring the same name with a different type.

You are actually calling operator<< on the ifstream , not the ofstream , hence the compiler error. The ofstream is out of scope by that time.

If you need a file stream that can be used for both input and output, use a single std::fstream variable instead, eg:

std::fstream hwidfile;
hwidfile.open("hardwareid2.txt", std::ios_base::in);
if (hwidfile.is_open()) {
    std::cout << "File exists\n";
    return 0;
}
hwidfile.open("hardwareid2.txt", std::ios_base::out);
if (!hwidfile.is_open()) {
    std::cout << "File create failed\n";
    return 0;
}
if (!(hwidfile << hwProfileInfo.szHwProfileGuid))
{
    std::cout << "File write failed\n";
    return -1;
}

Otherwise, give each file stream variable different names, eg:

std::ifstream hwidfileIn("hardwareid2.txt");
if (hwidfileIn.is_open()) {
    std::cout << "File exists\n";
    return 0;
}
std::ofstream hwidfileOut("hardwareid2.txt");
if (!hwidfileOut.is_open()) {
    std::cout << "File create failed\n";
    return 0;
}
if (!(hwidfileOut << hwProfileInfo.szHwProfileGuid))
{
    std::cout << "File write failed\n";
    return -1;
}

Or, simply get rid of the ifstream and use std::filesystem::exists() or equivalent instead, eg:

if (std::filesystem::exists("hardwareid2.txt")) {
    std::cout << "File exists\n";
    return 0;
}
std::ofstream hwidfile("hardwareid2.txt");
if (!hwidfileOut.is_open()) {
    std::cout << "File create failed\n";
    return 0;
}
if (!(hwidfile << hwProfileInfo.szHwProfileGuid))
{
    std::cout << "File write failed\n";
    return -1;
}