boost::optional vector pass by reference as default parameter

Question

boost::optional<std::vector<std::wstring>> filePath;

If I have the above boost optional vector is it possible to then pass this by reference and as an optional parameter?

Test(const boost::filesystem::path& targetPath, boost::optional<std::vector<std::wstring>> filePath = boost::none);

Could I pass filePath as a default parameter and by reference at the same time?

possible duplicate of [Default value to a parameter while passing by reference in C++](http://stackoverflow.com/questions/1059630/default-value-to-a-parameter-while-passing-by-reference-in-c) — BartoszKP, Jun 18 '15 at 10:35
There were compilation issues when i tried to do this with boost::optional @BartoszKP — CodersSC, Jun 18 '15 at 10:37
Please post these errors and a minimal source code that reproduces the problem. How can we guess what you did and what happened? — BartoszKP, Jun 18 '15 at 10:44
This is a valid question IMO. With an interesting answer. Hold on. — sehe, Jun 18 '15 at 11:25

score 4 · Answer 1 · edited Jun 18 '15 at 11:33

You can use an optional reference:

See http://www.boost.org/doc/libs/1_58_0/libs/optional/doc/html/boost_optional/optional_references.html

Live On Coliru

#include <boost/optional.hpp>
#include <boost/filesystem.hpp>
#include <vector>
#include <iostream>

void Test(const boost::filesystem::path& targetPath,
          boost::optional<std::vector<std::wstring>& > filePath = boost::none) {
    if (filePath)
        std::cout << filePath->size() << " elements\n";
    else
        std::cout << "default parameter\n";
}

int main() {
    std::vector<std::wstring> path(3, L"bla");

    Test("blabla", path);
    Test("blabla");
}

Prints

3 elements
default parameter

score 0 · Answer 2 · answered Jun 18 '15 at 10:32

0

What you are doing is legal, but you can't pass a reference as a default parameter. Should you want that, you'd need to pass a value, or the file path wrapped around another boost::optional.

answered Jun 18 '15 at 10:32

dau_sama

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But you can because `boost::optional` explicitly supports that – sehe Jun 18 '15 at 11:27
oh wow, I didn't know it supported it. you always learn something new – dau_sama Jun 18 '15 at 11:53

score 0 · Answer 3 · edited May 23 '17 at 12:29

0

Either put the reference inside the boost optional (boost::optional<std::vector<std::wstring>& >) as @sehe wrote, or use a const reference:

void Test(const boost::filesystem::path& targetPath,
          const boost::optional<std::vector<std::wstring> >& filePath = boost::none)
{

}

live example: http://coliru.stacked-crooked.com/a/324e31e1854fadb9

In C++ you are not allowed to bind a temporary to a non-const reference. In this case, the default value is the temporary so you need a const reference.

edited May 23 '17 at 12:29

Community

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answered Jun 18 '15 at 11:39

m.s.

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This might work depending on the requirements of the OP. If the vector needs to be non-const he's out of luck. If he uses the `optional const&` then toplevel const doesn't add anything (because `optional` is just fine). You might want to add an explanation why `const&` works. – sehe Jun 18 '15 at 12:05
So you explain how you can bind a const-reference to a temporary by... saying you can not do something else. Mmm – sehe Jun 18 '15 at 12:46

boost::optional vector pass by reference as default parameter

3 Answers3