Operator << overwrite with std::ostringstream issue

Question

Here's my toy program. It fails to compile with error message " no operator << matches these operands". Any help will be appreciated.

struct foo {
    std::string name;
};

std::ostringstream& operator<<(std::ostringstream& os, const foo& info)
{
    os << info.name;
    return os;
}

void insert(const foo& info)
{
    std::ostringstream os;
    os << "Inserted here " << info;  // don't work here
}

int main()
{
    foo f1;
    f1.name = "Hello";
    insert(f1);

}

R Sahu · Accepted Answer · 2019-03-23T05:00:35.740

5

The reason

os << "Inserted here " << info;

does not work is

os << "Inserted here "

returns a std::ostream&, not a std::ostringstream&.

Options:

Change your function to be use std::ostream instead of std::ostringstream.
```
std::ostream& operator<<(std::ostream& os, const foo& info) { ... }
```
Change how you use it. Use
```
os << "Inserted here ";
os << info;
```

I strongly recommend using the first option.

edited Mar 23 '19 at 05:00

answered Mar 23 '19 at 04:57

R Sahu

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In # 1, `std::stream& os` -> `std::ostream& os`. – Azeem Mar 23 '19 at 05:00

Operator << overwrite with std::ostringstream issue

1 Answers1