奇怪GCC錯誤：預期前主表達式「」令牌

Question

我仍然試圖從MSVC遷移到GCC，但我似乎無法找到解決如下問題：

template < typename A, typename B, typename C, typename D > 
class Test 
{ 
public: 
    Test (B* pObj, C fn, const D& args) : _pObj(pObj), _fn(fn), _args(args) 
    { 
    } 

    A operator()() 
    { 
     return _args.operator() < A, B, C > (_pObj, _fn); // error: expected primary-expression before ',' token 
    } 

    B* _pObj; 
    C _fn; 
    D _args; 
}; 

請幫忙！

Answer 1

嘗試return _args.template operator() < A, B, C > (_pObj, _fn);。

沒有template關鍵字，解析會有所不同。如果沒有額外使用template，編譯器不知道下面的小於號（<）並不是真的「小於」，而是模板參數列表的開始。

14.2/4

When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.

P.S：閱讀本Stackoverflow FAQ Entry