在实现平方根函数时候，可以检查系统是否带有某个函数，比如log，exp。用CheckSymbolExists模块里的函数在math.h中检查，如果有就用exp（1/2）来替换原来的函数实现。具体如下：
- MathFunctions/CMakeLists.txt中内容如下，
add_library(MathFunctions mysqrt.cxx)
target_include_directories(MathFunctions
? ? ? ? ? INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
? ? ? ? ? )
# does this system provide the log and exp functions?
include(CheckSymbolExists)
check_symbol_exists(log "math.h" HAVE_LOG)
check_symbol_exists(exp "math.h" HAVE_EXP)
if(NOT (HAVE_LOG AND HAVE_EXP))
? unset(HAVE_LOG CACHE)
? unset(HAVE_EXP CACHE)
? set(CMAKE_REQUIRED_LIBRARIES "m")
? check_symbol_exists(log "math.h" HAVE_LOG)
? check_symbol_exists(exp "math.h" HAVE_EXP)
? if(HAVE_LOG AND HAVE_EXP)
? ? target_link_libraries(MathFunctions PRIVATE m)
? endif()
endif()
if(HAVE_LOG AND HAVE_EXP)
? target_compile_definitions(MathFunctions
? ? ? ? ? ? ? ? ? ? ? ? ? ? ?PRIVATE "HAVE_LOG" "HAVE_EXP")
endif()
install(TARGETS MathFunctions DESTINATION lib)
install(FILES MathFunctions.h DESTINATION include)
- MathFunctions/mysqrt.cxx内容如下，
#include <math.h>
#include "MathFunctions.h"
#include <cmath>
#include <iostream>
double mysqrt(double x)
{
#if defined(HAVE_LOG) && defined(HAVE_EXP)
? double result = exp(log(x) * 0.5);
? std::cout << "Computing sqrt of " << x << " to be " << result
? ? ? ? ? ? << " using log and exp" << std::endl;
#else
? double result = x;
#endif
? ?return result;
}
编译与执行，
?
