Question

错误：

main.cpp: In function 'constexpr int fib(int)':
main.cpp:6:42: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     return (n < sizeof(ftbl)/sizeof(*ftbl)) ? ftbl[n] : fib(n-2) + fib(n-1);
                ^
/tmp/cch0aLwI.o: In function `main':
main.cpp:(.text.startup+0xf): undefined reference to `std::cout'
main.cpp:(.text.startup+0x16): undefined reference to `std::ostream::operator<<(int)'
main.cpp:(.text.startup+0x2d): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)'
main.cpp:(.text.startup+0x3c): undefined reference to `std::cout'
main.cpp:(.text.startup+0x43): undefined reference to `std::ostream::operator<<(int)'
main.cpp:(.text.startup+0x5a): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)'
main.cpp:(.text.startup+0x69): undefined reference to `std::cout'
main.cpp:(.text.startup+0x70): undefined reference to `std::ostream::operator<<(int)'
main.cpp:(.text.startup+0x87): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)'
/tmp/cch0aLwI.o: In function `_GLOBAL__sub_I_main':
main.cpp:(.text.startup+0xaa): undefined reference to `std::ios_base::Init::Init()'
main.cpp:(.text.startup+0xb9): undefined reference to `std::ios_base::Init::~Init()'
collect2: error: ld returned 1 exit status

它在clang中执行没有问题。

该例子来自B.Stroustrup新书“TCPL”的第312页。

#include <iostream>

constexpr int ftbl[] { 1, 2, 3, 5, 8, 13 };
constexpr int fib(int n)
{
    return (n < sizeof(ftbl)/sizeof(*ftbl)) ? ftbl[n] : fib(n-2) + fib(n-1);
}

int main()
{
    std::cout << fib(1) << '\n';
    std::cout << fib(2) << '\n';
    std::cout << fib(6) << '\n';
}

编译器调用：gcc-4.8 -std=c++11 -O2 -pedantic -pthread -Wall main.cpp && ./a.out

Answer 1

使用g++代替gcc。

或者，您可以根据需要链接标准库。由于链接顺序，你需要把它放在最后。

gcc-4.8 -x c++ -std=c++11 -O2 -pedantic -Wall -pthread main.cpp -lstdc++  && ./a.out

这个代码在gcc-4.8中发生了什么？

1 个答案: