cerr未定义

Question

我遇到了一些问题，我收到了这些错误（在代码中标明）：

• 标识符“cerr”未定义
• 无操作员“＆lt;＆lt;”匹配这些操作数

为什么？

#include "basic.h"
#include <fstream>

using namespace std;

int main()
{

    ofstream output("output.txt",ios::out);
    if (output == NULL)
    {
        cerr << "File cannot be opened" << endl;   // first error here
        return 1;
    }

    output << "Opening of basic account with a 100 Pound deposit: "
        << endl;
    Basic myBasic (100);
    output << myBasic << endl;   // second error here
}

Answer 1

您必须包含iostream才能使用cerr 见http://en.cppreference.com/w/cpp/io/basic_ostream。

Answer 2

您需要在顶部添加：

#include <iostream>

用于cerr和endl

Answer 3

包括用于cerr支持的iostream。

并没有运营商＆lt;＆lt;的实现对于Basic类。你必须自己做这个实现。见here.

Answer 4

#include <fstream>
#include <iostream>

#include "basic.h"


std::ostream& operator<<(std::ostream &out, Basic const &x) {
  // output stuff: out << x.whatever;
  return out;
}

int main() {
  using namespace std;

  ofstream output ("output.txt", ios::out);
  if (!output) {  // NOT comparing against NULL
    cerr << "File cannot be opened.\n";
    return 1;
  }

  output << "Opening of basic account with a 100 Pound deposit:\n";
  Basic myBasic (100);
  output << myBasic << endl;

  return 0;
}