附加一个空字符串 ostream 可防止进一步附加

Question

这个程序没有像我预期的那样工作。它输出“abc”，但我期望“abcxyz”。将包含空字符串的流附加到流中，会导致 streampos 设置为 -1，并且不能向其附加更多数据。 我正在使用来自C++ - Send data in one ostream to another ostream 的技术，但如果其中一个流恰好是空的，它就不起作用。我原以为在这种情况下 rdbuf() 可能会返回 nullptr，所以我检查了一下。它不是空值，但无论它返回什么都不会让下游感到高兴。注释掉该行：out << b.rdbuf(); 使其按预期输出“abcxyz”。附加可能碰巧为空的 ostream 的正确方法是什么？

#include <iostream>
#include <sstream>

int main(int argc, const char * argv[]) {
    std::stringstream a, b, c, out;

    a << "abc";
    b << "";
    c << "xyz";

    out << a.rdbuf();
                printf("after A out.tellp = %qd\n", (int64_t)out.tellp());
    out << b.rdbuf();
                printf("after B out.tellp = %qd\n", (int64_t)out.tellp());
    out << c.rdbuf();
                printf("after C out.tellp = %qd\n", (int64_t)out.tellp());

    std::cout << out.rdbuf() << std::endl;

    return 0;
}

输出是：

after A out.tellp = 3
after B out.tellp = -1
after C out.tellp = -1
abc
Program ended with exit code: 0

---

edit：固定版本。有没有更好的办法？

#include <iostream>
#include <sstream>

template <class T, class U>
void append(T& a, U& b) 
{
    if (int64_t(b.tellp()) <= 0) return;
    a << b.rdbuf();
}

int main(int argc, const char * argv[]) {
    std::stringstream a, b, c;

    a << "abc";
    b << "";
    c << "xyz";

    append(std::cout, a);
    append(std::cout, b);
    append(std::cout, c);
    std::cout << std::endl;

    return 0;
}

Answer 1

您可以使用std::basic_ios::good() 和std::basic_ios::clear() 来实现您想要实现的目标。

以下是示例代码。见it working here:

#include <iostream>
#include <sstream>

int main(int argc, const char * argv[]) 
{
    std::stringstream a;
    std::stringstream b;
    std::stringstream c;
    std::stringstream out;

    a << "abc";
    b << "";
    c << "xyz";

    out << a.rdbuf();
    printf("after A out.tellp = %qd\n", (int64_t)out.tellp());

    out << b.rdbuf();
    if( ! out.good() )
    {
        std::cout<< "It is not good!!!" << std::endl;
        out.clear();
    }
    printf("after B out.tellp = %qd\n", (int64_t)out.tellp());

    out << c.rdbuf();
    printf("after C out.tellp = %qd\n", (int64_t)out.tellp());

    std::cout << out.rdbuf() << std::endl;

    return 0;
}