您可以使用 -Wfloat-conversion 标志或更广泛的 -Wconversion 。

但是，请注意，使用C ++ 11 统一初始化大括号语法时，您会收到开箱即用的警告，但不包含 -Wconversion 标志;例如：

  #include< string> 

 double getSomeValue（）{
 return 4.0; 


 int main（）{
 std :: string str {getSomeValue（）}; // C ++ 11 brace-init 
} 
 

  C：\Temp\CppTests> g ++ -std = c ++ 11 test.cpp 
 test.cpp：在函数'int main（）'中：
 test.cpp：8：35：warning：缩小'getSomeValue（）'从'double'的转换时间t $ b $'char'内部{} [-Wnarrowing] 
 std :: string str {getSomeValue（） }; 
 ^ 
 

I recently had a bug in a similar context to next one:

double getSomeValue()
{
    return 4.0;
}
...
std::string str;
str = getSomeValue();

As you can see here is easy to spot the problem, but in a large code base where getSomeValue() is not in the same file with the calling code it might be difficult to spot this double to std::string silent conversion. GCC compiles this code fine with -Wall -Wextra -Werror (sample output here, I don't know what warning flags were used: http://ideone.com/BTXBFk).

How may I force GCC to emit warnings for these dangerous implicit conversions? I tried -Wconversion, but it is very strict and it causes errors in most included headers for common cases like unsigned - 1. Is there a weaker version of -Wconversion?

You can use the -Wfloat-conversion flag, or the broader -Wconversion.

However, note that with C++11 uniform initialization brace syntax, you get a warning "out of the box", without the -Wconversion flag; e.g.:

#include <string>

double getSomeValue() {
    return 4.0;
}

int main() {   
    std::string str{ getSomeValue() }; // C++11 brace-init
}

C:\Temp\CppTests>g++ -std=c++11 test.cpp
test.cpp: In function 'int main()':
test.cpp:8:35: warning: narrowing conversion of 'getSomeValue()' from 'double' t
o 'char' inside { } [-Wnarrowing]
     std::string str{ getSomeValue() };
                                   ^