为什么BOOST_TEST（（Iterator == Iterator））需要额外的括号？

Question

I am testing my implementation of binary search from binsearch.hpp :

template<typename Iterator, typename T>
Iterator binsearch(Iterator begin, Iterator end, const T &v) {
    if (std::distance(begin, end) == 0) {return end;}

    Iterator save = end;

    while (std::distance(begin, end) > 0) {
        Iterator mid = begin + std::distance(begin, end) / 2;

        if (*mid == v) {
            return mid;
        }

        if (v < *mid) {
            end = mid;
        } else {
            begin = mid + 1;
        }
    }

    return save;
}

with the following boost-driven unit test test_binsearch.cpp :

#define BOOST_TEST_MODULE test_binsearch
#define BOOST_TEST_DYN_LINK
#include <boost/test/unit_test.hpp>

#include "binsearch.hpp"

BOOST_AUTO_TEST_CASE(empty_0) {
    std::vector<int> xs = {};

    const auto result = binsearch(xs.begin(), xs.end(), 42);

    BOOST_TEST((result == xs.end()));
}

Unless I surround the comparison inside the BOOST_TEST with an extra pair of parentheses, I get a very cryptic and long compilation error that repeatedly tries to convert an iterator to a char, an error code and a few other types:

note: cannot convert 't' (type 'const __gnu_cxx::__normal_iterator std::vector >') to type 'char'

  ostr << t; 

I got the clue to surround my comparison with extra parentheses from here . Why does it fail to compile without them?

Answer 1

From the documentation of BOOST_TEST:

Complex statements

BOOST_TEST provides an enhanced reporting capability: additional details of the failing operands and operations are provided in the log , as shown on the example below:

Example: BOOST_TEST enhanced reporting ( reduced version )

Code

 #define BOOST_TEST_MODULE boost_test_macro3 #include <boost/test/included/unit_test.hpp> BOOST_AUTO_TEST_CASE( mytest ) { int a = 13, b = 12; BOOST_TEST(a - 1 < b); } 

Output

 > ./boost_test_macro3 --log_level=all Running 1 test case... Entering test module "boost_test_macro3" test.cpp(12): Entering test case "mytest" test.cpp(17): error: in "mytest": check a - 1 < b has failed [13 - 1 >= 12] ^^^^^^^^^^^^ !!! sic !!! ... 

Pay attention to the emphasized fragment of the test failure report. For those with moderate conduct of C++ it may seem like magic (newbies don't get surprised since they are not aware of the limitations of the language; experts know the secret or figure it out easily).

The documentation superficially explains the "magic" as follows:

BOOST_TEST parses the statement and constructs an expression out of it.

However, the "magic" doesn't work with types that cannot be "printed to an std::ostream ". Hence your compilation error. Fortunately the "magic" (which is based on operator overloading) cannot override C++ rules of operator precedence. By adding the extra pair of parentheses you define the territory where the "magic" cannot reach.