C++: Pass string literal as a string& leads to compilation failure, why

Question

I am using g++4.1.2 to compile this:

 cat 1.cpp #include<iostream> #include<string> using namespace std; void f(const string& s){} void g(string& s){} void h(string s){} int main() { string s="abc"; f("abc"); g("abc”);//Error h("abc"); return 0; }

It failed at line “g();”

$ g++ 1.cpp 1.cpp: In function ‘int main()’: 1.cpp:11: error: invalid initialization of non-const reference of type ‘std::string&’ from a temporary of type ‘const char*’ 1.cpp:5: error: in passing argument 1 of ‘void g(std::string&)’

I don’t quite understand that, as long as “abc” could be the constructor parameter for std::string:

For f()，"abc" can be used to construct a string(const char*), as a const reference to string

For h() “abc” can be used to construct a r-value, temporary std::string object.

For g()，why it cannot be used to construct a left-value reference string&, while const reference string& is OK?

You used a weird quote mark in the g call .. it should be " — M.M
– M.M, Commented Nov 11, 2016 at 2:12

krzaq · Accepted Answer · 2016-11-11 02:23:57Z

That's because the language rules are designed this way. You can bind an rvalue to a const lvalue reference (and extend its lifetime to that of the reference), because it is explicitly allowed.

Assuming C++03 since you're using g++ 4.1.2

§ 8.5.3 [dcl.init.ref] / 5

A reference to type “cv1T1” is initialized by an expression of type “cv2T2” as follows:

If the initializer expression

is an lvalue (but is not a bit-field), and “cv1T1” is reference-compatible with “cv2T2,” or

has a class type (i.e.,T2is a class type) and can be implicitly converted to an lvalue of type [...]

Otherwise, the reference shall be to a non-volatile const type [...]

If the initializer expression is an rvalue, withT2a class type, and “cv1T1” is reference-compatible with “cv2T2,” [...]

Otherwise, a temporary of type “cv1T1” is created and initialized from the initializer expression using the rules for a non-reference copy initialization (8.5). The reference is then bound to the temporary.

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