Runtime error for CLang compiled program (Mac) reading double type with std::cin

问题

I'm pretty new to C++ and working through some examples of the book "Programming Principles and Practices Using C++" (2nd Edition). I wrote the following simple Program (in file Main.cpp):

#include <iostream>
#include <string>

int main () {

    double d = 0;
    std::string s = "";

    while (std::cin >> d >> s) {
        std::cout << "--" << d << " " << s << "\n";
    }

    std::cout << "FATAL? "<< d << " " << u << "\n";
}

Compiling the program (on the command line) with CLang (Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn) Target: x86_64-apple-darwin13.2.0 Thread model: posix):

clang++ -o Main -std=c++11 -stdlib=libc++ Main.cpp

works fine without any errors. However, when I run the program it behaves strange. I tested the following input:

123m

which results in

--123 m

which is fine (the same holds for entering 123 m). But, entering the following:

123a

results in:

FATAL? 0 m

The same happens for most other characters (e.g. b, c, ...). Entering 123 a works fine though (output: --123 a).

Using GNU g++ works on the other hand. Further, the problem does not come up on a Linux machine compiling the same program with CLang.

As stated before, I'm new to C++ and this seems to be a Mac OS X specific problem. Is this a bug in the Mac CLang implementation or am I doing something seriously wrong here :(?

Thanks in advance!

回答1:

std::basic_istream::operator>> calls std::num_get::get to extract the value from input. Until C++11, the behaviour of std::num_get::get was like that of scanf with the appropriate formatting string. C++11 onwards, std::num_get::get ends up calling strto* functions, which have a more flexible matching than the one based on scanf. In your example, 123[a-f] get interpreted as hex. Since all the input has been consumed by >>d, the >>s part of while(std::cin >> d >> s) leads to the parse failing.

回答2:

Ok, I found a solution to the problem in this question asked: CGAL: How can I successfully compile and link CGAL examples (on Mac OS X 10.9 Mavericks)

Compiling with clang++ as follows:

clang++ -o Main -std=c++11 -stdlib=libstdc++ Main.cpp

instead of:

clang++ -o Main -std=c++11 -stdlib=libc++ Main.cpp

solved the problem.

Anyway, as libc++ should be the preferred library to use with clang++ (as I just was told offline) I think it's time for a bug report.

来源：https://stackoverflow.com/questions/24487481/runtime-error-for-clang-compiled-program-mac-reading-double-type-with-stdcin

标签

c++

c++11

osx-mavericks