英文:
How to get a compile time error in constant evaluated expression?
问题
我有一个用于评估断言的Assert函数：
-
如果前提条件在运行时失败，此函数将输出错误消息并终止程序。
-
如果前提条件在常量表达式内失败，它将导致编译时错误。
我希望当断言在常量求值表达式中失败时，这个函数也能生成编译时错误：
const int a = (Assert(false),0); // 生成运行时错误
// => 我希望它生成编译时错误
我考虑使用std::is_constant_evaluated：
#include <type_traits>
using namespace std;
void runtime_error();
constexpr void compile_time_error(){} // 应该生成编译时错误
constexpr void Assert(bool value){
if (value) return;
if (is_constant_evaluated())
compile_time_error();
else
runtime_error();
}
void func(){
const int a = (Assert(false),0);
}
我只使用GCC，我已经寻找了一个能够引发编译时错误且是constexpr的内置函数，但没有找到。
有没有什么窍门可以在可能被常量求值的表达式中引发编译时错误？
英文:
I have an Assert function that I use to evaluate assertion:
-
if the precondition fails at runtime, this function will output an error message and it will terminate the program.
-
if the precondition fails inside a constant expression, it will cause a compile time error.
I would like that this function also generates a compile time error when the assertion fails in constant evaluated expression:
const int a = (Assert(false),0); //generate a runtime error
//=> I would like it generates a compile time error
I thought about using std::is_constant_evaluated: compiler-explorer
#include <type_traits>
using namespace std;
void runtime_error();
constexpr void compile_time_error(){} //should generates a compile time error
constexpr void Assert(bool value){
if (value) return;
if (is_constant_evaluated())
compile_time_error();
else
runtime_error();
}
void func(){
const int a = (Assert(false),0);
}
I only use GCC, I have look for a builtin function that would cause a compile time error and that would be a constexpr but did not find one.
Is there any trick to get a compile time error in expression that could be constant evaluated?
答案1
得分: 5
你可以调用一个未定义的函数以引发编译时错误。或者，既然你正在使用gcc，你可以在常量部分内调用一个属性错误函数，以在编译此单元时引发编译时错误。要使其工作，你必须启用优化编译。
我看到在gcc 9.2中，使用std::is_constant_expression不起作用，但我成功地使用了__builtin_constant_p。
#include <type_traits>
constexpr void Assert(bool value) {
if (__builtin_constant_p(value)) {
if (!value) {
extern __attribute__(( __error__ ( "error" ) ))
void compile_time_error(void);
compile_time_error();
}
} else {
if (!value) {
void runtime_error();
runtime_error();
}
}
}
void func(int b) {
const int a = (Assert(false), 0);
Assert(b == 0);
}
我曾经写过一个名为curb的C库，可以执行类似这样的操作。
英文:
You can call a function that is nowhere defined to cause a compile time error. Or, since you are using gcc anyway, you can call a attribute error function from inside the constant part to cause a compile time error during compilation of this unit. To make it work, you have to compile with optimizations enabled.
I see that with std::is_constant_expression it does not work in gcc 9.2, but I managed it to work with __builtin_constant_p.
#include <type_traits>
constexpr void Assert(bool value) {
if (__builtin_constant_p(value)) {
if (!value) {
extern __attribute__(( __error__ ( "error" ) ))
void compile_time_error(void);
compile_time_error();
}
} else {
if (!value) {
void runtime_error();
runtime_error();
}
}
}
void func(int b) {
const int a = (Assert(false), 0);
Assert(b == 0);
}
I have once written a library in C I called curb that would do something like this.
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