Problem when constructing the lambdas with compiler of specific version

Question

Why this code snippet compiles with GCC 4.9.3,

#include<memory>
#include<string>
#include<cstdio>

int ExecCmd(const std::string& cmd, std::string& output)
{
    int ret = 0;

    const auto deleter= [&ret](FILE* file){
                              if(NULL==file)
                              {
                                  ret = -1;
                              } 
                              else 
                              {
                                  ret = pclose(file);
                              }
                            };

    {
        std::unique_ptr<FILE, decltype(deleter) > pipe{popen(cmd.c_str(), "r"),  deleter};
    }

    return ret;
}

int main()
{

}

whereas this code snippet does not compile with GCC 4.9.3：

#include <memory>
#include <iostream>
#include <string>
#include <cstdio>
#include<cstdio>

int ExecCmd(const std::string& cmd, std::string& )
{
    int ret = 0;
    
    auto deleter{[&ret](FILE* file){
                              if(NULL==file)
                              {
                                  ret = -1;
                              } 
                              else 
                              {
                                  ret = pclose(file);
                              }
                            }
    };
    
    {
        std::unique_ptr<FILE, decltype(deleter)> pipe(popen(cmd.c_str(), "r"), deleter);
    }
    return ret;
}

int main()
{

}

Here is what the compiler complains:

In file included from /opt/compiler-explorer/gcc-4.9.3/include/c++/4.9.3/memory:81:0,
                 from <source>:1:
/opt/compiler-explorer/gcc-4.9.3/include/c++/4.9.3/bits/unique_ptr.h: In instantiation of 'std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = _IO_FILE; _Dp = std::initializer_list<ExecCmd(const string&, std::string&)::<lambda(FILE*)> >]':
<source>:24:87:   required from here
/opt/compiler-explorer/gcc-4.9.3/include/c++/4.9.3/bits/unique_ptr.h:236:16: error: no match for call to '(std::unique_ptr<_IO_FILE, std::initializer_list<ExecCmd(const string&, std::string&)::<lambda(FILE*)> > >::deleter_type {aka std::initializer_list<ExecCmd(const string&, std::string&)::<lambda(FILE*)> >}) (_IO_FILE*&)'
    get_deleter()(__ptr);
                ^

Note:

I only have to use GCC 4.9.3.
I always use auto lambda{[](...){...}} to construct lambdas. This is the first time that I encounter such problem. And It makes dazed that should I never construct lambdas in this way in the future with GCC4.9.3? Or under what conditions, I should pay attention to such a problem?

Thanks to @J6t, @JeJo. I found how to make the code snippet complies.

I thought and thought, but I still can't find the answer.

Could anybody shed some light on this matter?

Maybe you could share the error message the compiler writes? — SimonC
– SimonC, Commented May 25, 2022 at 7:35
So, your question is why does auto deleter = [&..].... work, but auto deleter {[&...].... not? The const in the first version does not change anything. — mch
– mch, Commented May 25, 2022 at 7:42
gcc 4.9.3 has the same issue with the much smaller example auto x{1}; int y = x; - x is an initializer_list, not an int. This seems to be a bug in 4.9.3. The solution is to not combine auto with a braced initializer. (It's not clear why you would desire to do this with the lambda but not with ret.) — molbdnilo
– molbdnilo, Commented May 25, 2022 at 7:51
@molbdnilo Is this really a bug? It seems that 4.9.3 just does not implement N3922, which came after C++11. According to this document, it is a C++17 language future that was implemented in GCC 5. — Daniel Langr
– Daniel Langr, Commented May 25, 2022 at 7:55
@DanielLangr It seems to me that you are 100% correct, and it was a defect in the language, not in gcc. Sufficiently old versions of icc and clang do the same thing. — molbdnilo
– molbdnilo, Commented May 25, 2022 at 7:57

Daniel Langr · Accepted Answer · 2022-05-25 08:43:31Z

2

The problem with the second version is that, in C++11 and C++14, the following initialization:

auto var{ whatever };

yields the type of var to be of std::initializer_list<something>.

This changed in C++17 which accepted the N3922 paper. This paper was adopted by GCC since version 5. Your 4.9.3 thus deduces std::initializer_list, which is not what you want.

I always use auto lambda{[](...){...}} to construct lambdas.

Bottom line: Don't do this with GCC 4.9.3.

edited May 25, 2022 at 8:43

answered May 25, 2022 at 8:02

Daniel Langr

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