A18-1-6 - Unofficial Autosar C++ Guidelines Reference

Rule A18-1-6 (required, implementation, automated)

All std::hash specializations for user-defined types shall have a noexcept function call operator.

Rationale

Some of standard library containers use std::hash indirectly. Function call operator should be defined as noexcept to prevent container simple access from throwing an exception. Note: Consider own hash specializations to use standard library specializations combined with XOR (ˆ) operation as implemented by boost::hash_combine: seed ^= std::hash< decltype (v)>{}(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);

Example

// $Id: A18-1-6.cpp 311792 2018-03-15 04:15:08Z christof.meerwald $

#include <cstdint>
#include <functional>
#include <string>
#include <unordered_map>

class A
{
public:
A(uint32_t x, uint32_t y) noexcept : x(x), y(y) {}

uint32_t GetX() const noexcept { return x; }
uint32_t GetY() const noexcept { return y; }

friend bool operator == (const A &lhs, const A &rhs) noexcept
{ return lhs.x == rhs.x && lhs.y == rhs.y; }
private:
uint32_t x;
uint32_t y;
};

class B
{
public:
B(uint32_t x, uint32_t y) noexcept : x(x), y(y) {}

uint32_t GetX() const noexcept { return x; }
uint32_t GetY() const noexcept { return y; }

friend bool operator == (const B &lhs, const B &rhs) noexcept
{ return lhs.x == rhs.x && lhs.y == rhs.y; }
private:
uint32_t x;
uint32_t y;
};

namespace std
{
// Compliant
template <>
struct hash<A>
{
std::size_t operator()(const A& a) const noexcept
{
auto h1 = std::hash<decltype(a.GetX())>{}(a.GetX());
std::size_t seed { h1 + 0x9e3779b9 };
auto h2 = std::hash<decltype(a.GetY())>{}(a.GetY());
seed ^= h2 + 0x9e3779b9 + (seed << 6) + (seed >> 2);
return seed;
}
};

// Non-compliant: string concatenation can potentially throw
template <>
struct hash<B>
{
std::size_t operator()(const B& b) const
{
std::string s{std::to_string(b.GetX()) + ’,’ + std::to_string(b.GetY())};
return std::hash<std::string>{}(s);
}
};
}

int main()
{
std::unordered_map<A, bool> m1 { { A{5, 7}, true } };

if (m1.count(A{4, 3}) != 0)
{
// ....
}

std::unordered_map<B, bool> m2 { { B{5, 7}, true } };

// Lookup can potentially throw if hash function throws
if (m2.count(B{4, 3}) != 0)
{
// ....
}
}

Unofficial Autosar C++ Guidelines Reference

Rationale

Example

See also