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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:

  1. I only have to use GCC 4.9.3.
  2. 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?

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  • 2
    Maybe you could share the error message the compiler writes? Commented May 25, 2022 at 7:35
  • 1
    So, your question is why does auto deleter = [&..].... work, but auto deleter {[&...].... not? The const in the first version does not change anything. Commented May 25, 2022 at 7:42
  • 4
    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.) Commented May 25, 2022 at 7:51
  • 1
    @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. Commented May 25, 2022 at 7:55
  • 1
    @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. Commented May 25, 2022 at 7:57

1 Answer 1

Reset to default
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.

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