我试图通过交换使用/sf/ask/17360801/中的示例清除std :: queue .但是,由于"删除功能"错误,它似乎不适用于lambda比较器.
最小 工作的 失败的例子:
#include#include using namespace std; int main(){ typedef pair ifpair; auto comp = []( ifpair a, ifpair b ) { return a.second > b.second; }; typedef priority_queue< ifpair , vector , decltype( comp ) > t_npq; t_npq npq( comp ); //do something with npq. finish using it (without emptying it) and clear for next round t_npq empty( comp ); swap(npq , empty); }
编译
g++ -std=c++11 /tmp/test.cpp -o /tmp/o
我收到以下错误:
/usr/include/c++/4.8/bits/move.h:176:11: error: use of deleted function ‘main()::__lambda0& main()::__lambda0::operator=(const main()::__lambda0&)’ __a = _GLIBCXX_MOVE(__b); ^ /tmp/test.cpp:6:18: note: a lambda closure type has a deleted copy assignment operator
g ++ -v
Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.8.1-10ubuntu9' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9)
我有点好奇这里究竟发生了什么,但更重要的是我真的想知道如何使这项工作.
虽然lambda表达式的结果是可构造的,但它不一定是可移动的,当然也不是可复制的.我会通过使用std::reference_wrapper<decltype(comp)>
比较器对象来绕过这个问题:
typedef pair<int,float> ifpair; auto comp = []( ifpair a, ifpair b ) { return a.second > b.second; }; typedef priority_queue< ifpair , vector<ifpair>, std::reference_wrapper<decltype( comp ) >> t_npq; t_npq npq( std::ref(comp) ); t_npq empty( std::ref(comp) ); swap(npq , empty);
由于lambda表达式的完整类型信息由引用包装器保留,因此即使闭包不为空也应该有效,并且在可行的情况下,应该可以内联函数.