我一直很好奇为什么一个错误会导致编译器生成很长的错误消息列表。以下示例是GCC 4.8.1下vector<string>第100行NULL和main.cpp元素之间错误比较的结果：
> g++ -g3 -std=c++11 main.cpp functions.cpp
main.cpp: In function ‘int main()’:
main.cpp:100:24: error: no match for ‘operator==’ (operand types are ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ and ‘long int’)
if(args[1] == NULL) {
^
main.cpp:100:24: note: candidates are:
In file included from /usr/include/c++/4.8/iosfwd:40:0,
from /usr/include/c++/4.8/ios:38,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/postypes.h:216:5: note: template<class _StateT> bool std::operator==(const std::fpos<_StateT>&, const std::fpos<_StateT>&)
operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
^
/usr/include/c++/4.8/bits/postypes.h:216:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::fpos<_StateT>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:64:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_pair.h:214:5: note: template<class _T1, class _T2> constexpr bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
^
/usr/include/c++/4.8/bits/stl_pair.h:214:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::pair<_T1, _T2>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_iterator.h:291:5: note: template<class _Iterator> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
operator==(const reverse_iterator<_Iterator>& __x,
^
/usr/include/c++/4.8/bits/stl_iterator.h:291:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::reverse_iterator<_Iterator>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_iterator.h:341:5: note: template<class _IteratorL, class _IteratorR> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
operator==(const reverse_iterator<_IteratorL>& __x,
^
/usr/include/c++/4.8/bits/stl_iterator.h:341:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::reverse_iterator<_Iterator>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_iterator.h:1031:5: note: template<class _IteratorL, class _IteratorR> bool std::operator==(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&)
operator==(const move_iterator<_IteratorL>& __x,
^
/usr/include/c++/4.8/bits/stl_iterator.h:1031:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::move_iterator<_Iterator>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_iterator.h:1037:5: note: template<class _Iterator> bool std::operator==(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&)
operator==(const move_iterator<_Iterator>& __x,
^
/usr/include/c++/4.8/bits/stl_iterator.h:1037:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::move_iterator<_Iterator>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/string:41:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/allocator.h:128:5: note: template<class _T1, class _T2> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_T2>&)
operator==(const allocator<_T1>&, const allocator<_T2>&)
^
/usr/include/c++/4.8/bits/allocator.h:128:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::allocator<_CharT>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/string:41:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/allocator.h:133:5: note: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
operator==(const allocator<_Tp>&, const allocator<_Tp>&)
^
/usr/include/c++/4.8/bits/allocator.h:133:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::allocator<_CharT>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/string:52:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/basic_string.h:2486:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
^
/usr/include/c++/4.8/bits/basic_string.h:2486:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: mismatched types ‘const std::basic_string<_CharT, _Traits, _Alloc>’ and ‘long int’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/string:52:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/basic_string.h:2493:5: note: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::basic_string<_CharT>&, const std::basic_string<_CharT>&)
operator==(const basic_string<_CharT>& __lhs,
^
/usr/include/c++/4.8/bits/basic_string.h:2493:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: mismatched types ‘const std::basic_string<_CharT>’ and ‘long int’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/string:52:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/basic_string.h:2507:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)
operator==(const _CharT* __lhs,
^
/usr/include/c++/4.8/bits/basic_string.h:2507:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: mismatched types ‘const _CharT*’ and ‘std::basic_string<char>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/string:52:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/basic_string.h:2519:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
^
/usr/include/c++/4.8/bits/basic_string.h:2519:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: mismatched types ‘const _CharT*’ and ‘long int’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/locale_facets.h:48:0,
from /usr/include/c++/4.8/bits/basic_ios.h:37,
from /usr/include/c++/4.8/ios:44,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/streambuf_iterator.h:204:5: note: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
^
/usr/include/c++/4.8/bits/streambuf_iterator.h:204:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::istreambuf_iterator<_CharT, _Traits>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/vector:64:0,
from main.h:16,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_vector.h:1403:5: note: template<class _Tp, class _Alloc> bool std::operator==(const std::vector<_Tp, _Alloc>&, const std::vector<_Tp, _Alloc>&)
operator==(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
^
/usr/include/c++/4.8/bits/stl_vector.h:1403:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::vector<_Tp, _Alloc>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/x86_64-suse-linux/bits/c++allocator.h:33:0,
from /usr/include/c++/4.8/bits/allocator.h:46,
from /usr/include/c++/4.8/string:41,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/ext/new_allocator.h:139:5: note: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
^
/usr/include/c++/4.8/ext/new_allocator.h:139:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const __gnu_cxx::new_allocator<_Tp>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_iterator.h:811:5: note: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
^
/usr/include/c++/4.8/bits/stl_iterator.h:811:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const __gnu_cxx::__normal_iterator<_Iterator, _Container>’
if(args[1] == NULL) {
^
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from main.h:13,
from main.cpp:12:
/usr/include/c++/4.8/bits/stl_iterator.h:805:5: note: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
^
/usr/include/c++/4.8/bits/stl_iterator.h:805:5: note: template argument deduction/substitution failed:
In file included from /usr/include/unistd.h:226:0,
from main.h:20,
from main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const __gnu_cxx::__normal_iterator<_IteratorL, _Container>’
if(args[1] == NULL) {
^
这可能会让人很难理解这个问题。这种行为的原因是什么？
答案 0 :(得分：4)
如果您的表达式为vec == NULL，则编译器应用ADL来查找要调用的相应operator==。如果它找不到一个，它会输出在所考虑的所有命名空间中找到的所有 operator==个候选项，以及它们不适合的原因。
其中一个看起来像这样：
In file included from [..] main.cpp:12:
[..]:216:5: note: template<class _StateT> bool std::operator==(const std::fpos<_StateT>&, const std::fpos<_StateT>&)
operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
^
[..]:216:5: note: template argument deduction/substitution failed:
In file included from [..] main.cpp:12:
main.cpp:100:27: note: ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ is not derived from ‘const std::fpos<_StateT>’
if(args[1] == NULL) {
^
我们无法将第一个参数与第一个参数匹配，因为std::string不是来自任何fpos - 特化（同时std::string没有任何合适的转换运算符，但这里没有提到）。如果您定义了无法找到/调用的operator==，您不想从编译器视图中了解原因吗？ （特别是如果你自己没有线索）
这有时非常有用。如果不是，请忽略它。在大多数情况下，C ++ - 程序员会立即看到并理解导致错误消息的原因（及其enourmos note - 部分）。
答案 1 :(得分：1)
这可能有助于此类情况（您收到大量与模板相关的消息） 无论您是使用make还是直接使用编译器，请尝试将stderr传递给我的示例中的文件
make 2＆gt; error.log中
使用任何编辑器打开error.log文件。搜索您编写的源代码的文件名。应该只有一两行对你感兴趣。例如：
mysourcefile.cpp:51:20: required from here
这就是问题发生的地方;在这里看起来更难。